As far as humans are concerned, the sole prospect of something to be gained is enough. That something doesn't even have to be found on their world itself. It might be a welcome distraction for domestic problems. If there is population pressure, there WILL be war, no matter how far advanced a society (at least where humans are concerned, where the rule that the IQ of a mass is that of its dumbest member divided by two applies without mercy). If kicking some aliens but is the only way of a gouvernement to satisfy a raging mob at home that isn't really interested in any plausible facts, they'll do it.But why? What do we, or an alien society less advanced than ourselves, have to offer? This isn't the Wild West, it's space.
The only viable way to make it happen would probably be a mcguffin (like Avatar's unobtainium). And even in fiction, it doesn't make sense.
It might happen the other way around, also.
Now, I don't want to be overly pessimistic here. Unless there pops up a reason for one of the two to step into closer relations than just stretching of their fingers in a fancy way, things would probably go smoothely. As soon as there is envy, greed or fear involved, things go to hell. At least as far as humans are concerned. And they get afraid awfully fast. Or can be made afraid awfully fast, should anyone have an interest in it.
I guess my point is, if their psychology works anything like ours, we're screwed...