They make you angry, but they also make you laugh. They drive you nuts on a daily basis, and still, you're crazy about them.
Also, you co-adopted a puppy last year and it would be really hard on her if you two split up.
The puppy is what psychologists call a "material constraint"; other examples include a house you co-own, a joint bank account, or vacation plans you've already paid for. Research suggests that material constraints make a breakup a lot less likely.
In fact, according to a 2011 study of unmarried men and women in heterosexual relationships, adding just one additional material constraint is linked to a 10% increase in a couple's chances of staying together.
What's especially interesting here is that the 2011 study found material constraints made a breakup less likely regardless of how dedicated the participants said they were to their partners. In other words, it's not necessarily that material constraints reflect couples' strong feelings for each other, and that's really why they end up staying together.
Instead, it's possible (though not certain) that couples wind up staying together because of "inertia": They're already sharing a home, or a gym membership, or a bank account, and navigating the politics of a breakup would be hard.
Other, more recent research, cited on New York Magazine, suggests that people can fall prey to the "sunk cost effect" when it comes to marriage. Specifically, the study found that people say they'd be more likely to stay in an unfulfilling marriage if they'd invested time, money, or effort in the relationship - for example, if they'd invested all their money in buying a house with their partner.
None of this is to say, of course, that whether you're married or unmarried, you should live in fear of moving in together or buying a house with your partner. You shouldn't.
But especially if you're not yet married, you might consider first discussing your thoughts about the future of the relationship. While research suggests that living together tends to predict likelihood of unhappiness and/or a breakup, psychologists say that's not true if the couple only starts living together after having "clear, mutually understood plans" to get married.
Perhaps most important, it's wise to think (on your own) about what you'd do if the relationship didn't work out. There's no one, right answer. But preparing in advance, even for a highly unlikely outcome, can save you at least some inner turmoil if it should ever come to pass.