Hi there, everyone.
I'd recommend Robert Wright's The Moral Animal for a highly entertaining survey of the evolutionary reasons for love, hate, sense of justice, etc. One of the most interesting points in the book is that natural selection fits us with "mental organs" - tendencies and feelings - that tend to maximize inclusive fitness. The carrier need not be aware of the underlying "logic."
So, for example, people and animals love their offspring without thinking about the logic behind it, even though there is clearly an important evolutionary reason. Natural selection gets us to behave certain ways by causing us to feel certain ways. Many species demonstrate things like friendship-based alliances, anger at injustice, and so on. They need not be aware of the particular function of these feelings, but the feelings spur them to behavior that increases their fitness.
For example, making deals with trustworthy reciprocal altruists and cutting off cheaters is a smart strategy. But a primate doesn't need to consciously think, "By golly, if I keep losing resources to that cheater, it will decrease my ability to effectively raise offspring!" Instead, it just feels anger at being cheated, and that feeling causes him to cut his losses.
SNG