Even before that, Cloud doesn't really -not- have a reason.
Since he thinks he's Zack and all, he has been massively betrayed by Shinra -- he's pretty much a fugitive so he's kinda gotta make ends meet with terrorist groups like Avalanche -- I always kinda saw his taking jobs with Avalanche (and staying, despite the pay being uncertain etc) as kind of an implicit anti-Shinra thing, once you learn of his past and such.
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You saved science, SuperNiceDog!
Congratulations to the Kentucky Wildcats, NCAA basketball champions!