Reasons for a party to form and stay together?

[Jasper Flick]

"You're the Red Shield Gang. You've always adventured together before play begins and you'll always be adventuring together even after play ends."
Works like a charm if people buy into it. The whole party thing needen't be an issue if you're fine with a bunch of characters doing one adventure after the other, slaying monsters and taking their stuff. I once used "You happen to be together when things go FUBAR and you'll only separate when you die." It was Gamist throught and through.

If you want something less shallow I agree with all who opt for making character creation a group effort.