At Web Summit in Dublin on Tuesday, CNN's Laurie Segall asked Houston to clarify what two things mean.
Houston said the tennis ball reminds him to work on a business he's obsessed with - like a dog who is focused on retrieving a tennis ball. He once started an SAT prep company and lost interest; he later realized it wasn't a problem he cared to spend a lifetime solving.
"For any of you who had dogs, you know what a dog chasing a tennis ball looks like, but nothing will get in the way," Houston told Segall. "Finding something you're obsessed with. The founders of these companies who go for a long time are solving problems they're obsessed with. That for me was the start of Dropbox."
The number 30,000 symbolizes the number of days are in the average person's life. By the time Houston realized that he was already down 8,000 days.
"I had just moved from Boston to San Francisco. I couldn't sleep and I was in my room reading and I read a sentence that said, 'You're going to live for 30,000 days,'" Houston said. "Every day you're tearing off a page of the calendar. Then I went to my calculator. I was 24. I was like, "Oh my God, I'm like 8,000 or 9,000 down. It's not like 1,000. I'm almost one-third through my life.'"
Another thing he'd tell himself is you can learn a surprising amount of information in five years. Houston used to search for the top books on Amazon in marketing, sales and business and read them all. He's also learned that some of the best business leaders in tech were engineers first and taught themselves business on the job. Houston believes it's very hard to do the reverse and be a businessman who scrambles to get technical.