Obviously they are jealous that we've had the time to come up with creatively delicious ways to enjoy our life while they've been doing...whatever it is they're doing. Okay, the Fallen might have a reason but the Cabal, those space rhinos just do war for the hell of it, I'm sure. If they're not picking on someone else they're beating on each other. The Vex? Eh, robot hivemind that just wants to overwrite everything, guess it's fair when we're part of the everything but we can still take offense. And then we have the Hive but let's just not get into the Hive.