How come whenever I play Halo.......?

This sounds kinda weird but whenever I play the video game Halo (usually 1 or 3) I always get really sad when I lose a Marine(s) in combat. I pretty much just replay campaign missions when I don't have anything better to do but whenever the Covenant s kill my Marines I always get really sad. I think i might be getting into the game too much. Why do I care so much when my Marine allies get killed?