So have you played Crusader Kings II? That's a pretty complex system, with a number of interesting variables and interactions (character traits, memory of past actions, etc.) So for example, if we're different religions, there's some distrust or enmity, but it's a much bigger issue in my feelings for you if I have the "zealous" trait. No one's especially concerned that I'm lusty, except for chaste people. Ambitious people have an automatic dislike of their superiors.
Nonetheless, this is still just boiled down into a -100 to 100 number. Like jbadams said this could be made more realistic. For example, a character might be loyal to his cousin when it comes down to war, and indeed love him, but think he's an idiot and not want to give him a county to rule.
Anyway, something like the CK2 system where the different traits move the relationship along different axes (from love to hate, from trust to distrust, etc.) would be interesting, but not (I think) conceptually too hard for a player.