There are a lot of board games that can be broadly described as historically-informed world conquest: Advanced Civilization, Seven Ages, Age of Renaissance, History of the World, and no doubt some set in modern times that I haven't played (like Diplomacy and Axis and Allies). In many of these you play a single empire throughout the game, so it's natural to feel a little "territorial" -- when someone attacks you it's an attack on your guys, and one often retaliates, leading to a more directly-compeetitive game. (As compared to, say, Euro Rails, where you're all competing but you're sharing the world.) Different players have different styles, so in my experience these games start with some attempt at agreement on the level of aggression, and a player who is either overly aggressive or overly cooperative and accommodating can screw up the mood for everyone.
Within the last few years I've been introduced to two games where you play multiple empires over the course of the game. You would expect this to lead to less player attachment; if one empire tanks, well, you've got the means to start another. In practice, I've found this to be true in one and not in the other.
In the few games of Seven Ages that I've played, I've noticed that players (at least in my group, and certainly this is true of me) still feel pretty attached to the empires they have in play. I might have three empires in play, each one bringing me points each turn, but if someone wipes one of them half off the board I feel unhappy, and thus I am more reluctant to do it to others. (I am more of a cooperative player than an aggressive one.) I say this knowing full well that (1) it's nothing personal, (2) I like this game, and (3) it's a game and you're supposed to do stuff like that.
In History of the World you also play several empires over the course of the game, and I realized this weekend while playing that the ebb and flow, rise and fall, conquest and obliteration is perfectly natural and not at all bothersome beyond the "hey, I was getting some points from those guys!" reaction. In this game if I need territory I take it, and I know others will do the same thing, and that's cool -- I've got more guys where those came from. So, what's the difference in this game? I think there are two key factors: predictability of new empires and timing of scoring.
In Seven Ages, you can only bring in a new empire when (1) you have pieces available and (2) you have an appropriate card. I've had games where I would have happily replaced a failing empire, but I did not have the means to do so for a few turns running. So I was stuck playing a losing position while waiting for the luck of the draw. Bummer. In History of the World, on the other hand, you play each empire for one turn, and then everyone gets another (not necessarily playing in the same order). Your residual empires help your score as long as they stick around, but there's this sense that everyone's moving on. You can't go back and modify those empires any more; they've been and gone. If I need to knock over your guys to build mine, well, those are the breaks. Sure, there's competition for resources, but it's more asynchronous. I also know that due to the variation in turn order, you might, or might not, go twice before I go again.
The other factor is the timing of the scoring. In Seven Ages, as in most games, everyone takes a turn and then you score everyone (or you score only at the end of the game, in some games). I can build my board position all I want, but it's only what survives after you're done that helps me ("all that work for nothing", possibly). In History of the World, each player plays and then scores before play passes to the next person. In this most recent game, in the last epoch every single player save one dominated northern Europe -- obviously not simultaneously. I went early that round, collected my points for it, and watched empire after empire squish my guys (and each other's, as the epoch went on) to build that position. It made it more natural for people to actually take the logical, historical conquest paths, and hey, I already got my points so it didn't take anything away from me directly -- there was just the general vying for getting the best overall score possible. In this sense, the scoring feels more like Euro Rails (make a delivery, get paid) than like Seven Ages. I'm finding that I like this model, though I don't necessarily dislike the other.