I love it when games pretend to have feelings. It's fun in its most immediate moments, like when a berserker charges at a loud noise or random mooks dive away from grenades, but the most entertainment comes from the long-term simulated simmering hatreds. The "Don't settle any more cities near me"
in Civilization or repeatedly offering Dwarf Fortress's life-loving Elves meat and wood products until they lay siege to your fort.
It's a matter of scale, too, I suppose. Aggravating an entire nation into a war you've already prepared to win tickles the tactical part of your brain. Or it should. I don't know your brain. Computerized opponents making their 'feelings' clear also helps soften the blow of their actions. Greece's sudden but inevitable betrayal feels a bit justified if it's made clear that you built their wonders, stole their lands, and these actions are not what Greece wants.
Even the barest of motivations give games some personality. It doesn't have to involve any fancy justification or provocation from the player, though those are great for immersion. Just slap a few angry faces and sound clips on top of whatever aggression behavior already exists. What's that? Your enemies are way too simple to justify an 'angry' personality? I have a few words for that.
BEWARE I LIVE
Sinistar just moves toward you. And taunts you. Then eats you. Then laughs. Sinistar is little more than a Pac-Man ghost with a better paint job. But that paint job is a face that hates you. Sinistar isn't some mindless zombie that wants to eat your face. Sinistar is an active and unavoidable evil that will stop at nothing to end you.
Because he has sound bites that make him angry.