When I first started working in film, I worked on a lot of horrible films with horrible people, horrible hours, and horrible pay, just to get experience under my belt. So maybe one reason would be, to get experience?
I also recently/finally ended a relationship which started out okay and went bad and kept looking like it would shape up but obviously didn't. So maybe another reason would be, because you thought it would turn out okay and you kept thinking that if you just behaved differently, you could magically change things to make them okay?
Or maybe you didn't know any better?
Like everyone else has said, being in a band can be so similar to other situations in life...relationships, careers, anything which involves connecting with others and taking a risk. It's a really unique and often precarious relationship, in some ways deeper than relationships, because it's so much harder to get 3-5 people to all get along, let alone a couple. It's also harder than a job, in some ways, because of the fact that so much of it boils down to "love of the game" and a degree of personal sacrifice which might be unthinkable in most jobs. At the least, there's no Human Resources department!
And like everything else, we eventually learn which bands and situations to avoid. After all, what's the definition of insanity?
Last winter I was in a startup band that was disguised as a "rebirth" with a "new lineup." It was fun at first because I felt there was good chemistry and I was excited to be more involved with the songwriting/production process. But it wore thin when tension arose between the bandleader and me (he...had a thing for me, I just wanted to be friends, story of my life), and then when we both started to feel at odds with the drummer, and we lost our guitarist, and....pretty soon it just wasn't worth it, especially when I started a new job and had to really work hard to manage my time without going crazy.
After I quit, I realized how much it bothered me to be in a startup project because I felt sort of strung along and like the logistics/scheduling end of things could've been handled so much better...yet I couldn't do anything about it, because it wasn't my band. I could make suggestions, but all that did was make me more responsible for something that I suddenly wasn't sure I wanted to be a part of. So, yeah. I took a big step back and realized that maybe I had to think harder about what sort of direction I want to go in a musician.
I dunno. Things change...people change...that's about it. All we can do is learn from it all!