I don't feel like I have to force my player's to generate a reason why they would all be working together. Rather I start with the overall plot of the game I want to run; without sharing said plot, I ask the player's to generate a backstory for their players (of course if some of them want to know one another beforehand, that's great; also, I at least share basic information such as campaign setting and specific region thereof). I then give them feedback regarding their backstory, sort of my take on it, complete with deatils (secret hints) about the upcoming game. If I have then modified their background in a way that they don't approve of, I give them a chance to ammend what I have done. The long and short of it is, there is a lot of back and forth betwen myself and my palyers regarding character background before we ever sit down to play.
So, this gives us the principle plot and character background. I then try to use the finalized (or as final as it gets before play begins) background to generate 1-2 subplots involving the characters that I can throw up as they make their way through the principle plot.
This may be a bit off topic, but I really strive to create plots and subplots that bring the characters into conflict with one another. I love to see how far I can push them without resorting to intraparty violence, then I back off the stressors just a bit. It's really quite a lot of fun and generates the best role-playing experience in my mind.
That said, it's not for everyone. You have to have a mature group to keep from things dissolving into anarchy. By mature, I guess I mean you have to have players that enjoy role-playing/character acting, but who will, when push comes to shove, put the good of the game above character integrity. Maybe mature is a bad word for this. For example, most fo my group (and it's a large group) loved this style of play, but one guy snapped and started yelling at other players (and I don't think he was "in character" at the time).
Also, creating a tense situation within the party is not for every game. I was running a modified Forgotten Realms game (lower than baseline FR magic, pulp action, Lovecraftian horror), so the intraparty tension/conflict was great. For a more light-hearted, heroic fantasy game, I'd probably shy away from it.
Enough of that.
Chad