Why It's So Incredibly Depressing To Be A Lawyer
Crippling student debt
Most countries have a way to limit the number of attorneys. Japan and China for instance have much harder bar exams with much lower passage rates. The U.S. does it by making us spend THREE YEARS and a ridiculous amount of money (Columbia was about 260-270k all in including room and board). This narrows your career options (i.e. why over 90% of my classmates from CLS in 2010 went into the type of biglaw firm where anon above [1] is a midlevel associate). If you've read his answer, you understand why some of my classmates are unhappy.
Law is depressing and lawyers are paid to be cynical
In law school, you learn about unjust laws and cases. After school, litigators are paid to resolve conflict — often between two acrimonious and irrational sides. Most of the conflicts that necessitate getting a lawyer are not happy ones. In other words, time and time, case after case, you're brought in after things have already gone horribly wrong. In many cases, you're helping make the situation even more wrong (e.g., expensive, protracted lawsuit that causes both sides extended grief and suffering).
Even corporate/contract lawyers are paid to think of all the things that could go wrong. The role of corporate and in-house counsel is to tell the business people the legal constraints and risks they face. Often, we're seen as the ones trying, at worst, to jettison a deal or, at best, getting in the way of a deal with our endless bickering over the ramifications of (in the eyes of the business folk: remote) possibilities.
I-bankers, and certain other finance types, have been known to call us "monkey scribes."
Law is boring
Finality, consistency, and efficiency/administrability are each just as important as fairness when it comes to what makes a law good or bad [2].
Unfortunately for lawyers, the desirability of having laws and cases that settle disputes with finality and consistency means that judges try to follow precedent, and even contract lawyers are usually paid to apply the law the same exact way every time. A significant amount of research assignments for corporate junior associates involve researching past cases and deals with similar fact patterns or contract provisions that are desired by our side on the current deal. Many conference calls with the other side's counsel degenerate to seeing who can cite more deals to justify that their version of the purchase agreement provision is "market" [3]
Law involves a lot of downtime and a lot of stop and go for both litigators and corporate folks (i.e., periods of high stress and activity followed by periods of having to be on call but not really having anything to do). This happens at the courthouse, the boardroom, in the office, and at the printers at ungodly times of the morning. This is beautifully described by anon's answer below already. I just stress that the lifestyle is incredibly unhealthy, mentally and physically, and has made many of my colleagues more prone to substance abuse and addiction [4].
Self-selection
This is one that I haven't seen others confess. The qualities I describe above: unhappy, bored/boring, and low financial means seem to fit most of the law students and lawyers I know.
I suppose it makes sense. Many unhappy, unfulfilled people — many of whom do not know what they want to do or are looking for a higher paying job — go to law school. Families that are obscenely wealthy are more likely to send (i.e., pay for) their children to get humanities PhDs than JDs when it comes to post-graduate education — certainly my wife's colleagues and classmates in Yale, Harvard, or Beida/Fudan's anthro and sociology PhD programs came from wealthier families, on average, than my classmates and colleagues from Yale law, HLS, CLS, etc.
Some people go to law school because they "like to argue." To be honest, I suppose being a douchebag may have made me want to go to law school a little bit more too. Assholes want a leg up on the competition when it comes to picking and winning fights.
Of course, since law and the practice of law have the same qualities (boring, depressing, money-hungry), these traits tend to get reinforced in law school and during practice. I've seen stick-thin humanities majors straight out of college with gentle eyes and old souls emerge from law school basements and law firm conference rooms after a decade of too much sitting, stress and seamless as enormous, immature lunatics who waddle along the halls of the firm raving obscenities at opposing counsel, clients, colleagues, subordinates, family, and lovers alike [5].
Clients/service industry From one of my earlier answers [3 again]
Law it is at the end of the day, a service industry with the expected levels of ass-kissing, unreasonable demands and advice that is solicited and then ignored. Much of law is cleaning up (or trying to prevent) the messes of others, and some of the time, especially at a junior level, you're just doing menial tasks that the client (i.e. in-house counsel) can't be bothered to do themselves. There are midlevel associates in biglaw who bill out at $500-700 an hour who spend a shocking amount of time doing stuff like fixing typos in disclosure documents and press releases.
To expound on the service industry aspect, there is tremendously less ownership over what is done with your work product and less opportunity to see the results of that work product. Reading about the deal you helped make happen on the front page of the WSJ doesn't mean you'll be reaping any additional profits or feeling any additional satisfaction if the deal is deemed successful in the coming few years. You won't even get overtime pay or a bonus for all the weekends and all-nighters. There also are the typical and expected agent-principal conflicts/problems between client and attorney.
Billing Already been mentioned here and elsewhere already [6]:
Billing entails keeping track of how many minutes you work for each client, whether you worked during travel time and if so how to apportion the time spent on the plane/train/etc. between the client you're traveling for and the one you did work for, and all the stupid little rules that differ widely from client to client (e.g., whether overtime meals or other expenses can be billed, whether they want you to bill in increments of 6 minutes or 15, whether they will pay for the work of a summer associate, whether they will pay for research, and so on). I've had days where I billed time to over a dozen different clients. It's not fun keeping track of all these things when you have a hundred other, more important matters on your mind.
For biglaw corporate lawyers who bill out at $500 to $2,000+ per hour, whether you count the 5 minutes you spent sitting on the can (but you were thinking about the deal/case the whole time!) as billable time can actually make a hundred dollars or so difference to the client. If my "diaries" slip behind by more than a couple weeks, I don't get my next paycheck. For sole practitioners or boutique firms who deal directly with smaller clients and who are under greater pressure to tailor their fee structures, it must be even worse.
Every lawyer more or less has to do this. You can have your secretary do some of it for you, but ultimately it's your responsibility.
Part of the stress is because every lawyer knows it is a slippery slope. One day you're padding your time by five or 10 minutes because you know other lawyers at your firm who do far worse, and the next day you're just making your timesheets up as you go and committing fraud and malpractice.
As someone who has worked for everything from construction crews to hedge funds and who returned to a general management and marketing role after two years in biglaw, I would note that no job is perfect. The practice of law does have positives. There are brilliant moments of triumph and self-actualization as well as the defeat and ennui described by the other answers here. If the question had been "why are lawyers happy with their jobs," I'm sure you would have gotten some good answers too.
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[1] Anonymous' answer
The comments are generally spot on as well.
[2] http://www.quora.com/Why-is-ther...
[3] Lack of innovation and creativity. Models/form agreements and other precedents (in both litigation and corporate) are given great deference, and even archaic turns of phrase are painstakingly preserved just in case changing the wording might change the legal meaning of a document [2]. On practically every M&A and securities agreement I've worked on, a partner, client, banker or someone from the other side will ask "What's market?" or "What was done on the ___ deal?" From a social and business point of view, this is understandable given the desirability of consistency in how law is practiced and applied, but it makes the job really boring after a while.
Attorneys: What is the most frustrating thing about being a lawyer?
[4] http://abovethelaw.com/2012/10/w...
Not all deliverymen standing in the lobbies of investment banks, PE shops, and biglaw firms past 8 pm on weeknights are delivering food.
[5] See 6 and 9: http://www.eharmony.com/dating-a...
That eharmony list was obviously not written by a lawyer, someone who has dated one, or anyone who has ever worked for or with one.