What makes people quit?

I was surfing around and found an old doubling blog. This girl, Rachel, managed to double her way from a penny to $1500 in less than a year. That's more than all the doubling I have been doing for three and a half years. She was WAY in front of any other doubler and then all of the sudden - out of the blue she disappeared. Her blog has been silent for two and a half years. Now I'm not judging - I've certainly had my share of extended leaves, and I don't know her life - maybe life got busy or maybe something really bad happened. I'm not trying to focus on Rachel, specifically, but rather explore why people get excited about a project for as long as a year and then it just fizzels out. Previously I had decided to make achievable due dates for the doubling, which seem to be working out (at least at this level). But what makes someone so passionate and then BAM, it just dies? Perhaps the reward seems to small for the effort, or perhaps life gets in the way. Maybe personal interests change? I'd really like to know because I really want to succeed at this - I've been doubling so long, I can't imagine ever stopping. Maybe it's just a case that slow and steady wins the race and I just have to keep plodding along and not worry so much about others dropping out of the race.

Flip