The current relationship system classifies dwarven relationships as friends, aquaintances, lovers, grudges, family members, and so on. The system is neat, but simplistic and inflexible. I propose that a system where a dwarf's relationship with another dwarf has two different variables:
The first, "feeling" (for want of a better term), describes how the dwarf feels towards the other dwarf, ranging from "love" to "like" to "neutral" to "hate", differentiating between romantic and platonic love. The variable will be adjusted accordingly to not just how compatible the personalities are and how much they come into contact with each other, but also their actions. Dwarfs will like each other less if say, dwarf A kills dwarf B's father, a furnace operator melts down a weaponsmith's masterpiece steel axe, or if a dwarf A's lover falls for dwarf B.
The second, "relationship", describes the type of relationship the two dwarfs share. This is similar to the current system. Non-familial relationships, like friendship and romance, will be affected by the first variable.
The advantage of this new system is that a dwarf is now capable of becoming hateful towards his spouse, his siblings, his parents, etc. How he feels about another dwarf can be independent of his "official" relationship with that dwarf. One-sided friendships and romances are also possible.
When a sufficient level of hatred is present, a dwarf will get an unhappy thought from the mere sight of the one he dislikes. He may start a fistfight with his nemesis, and may even make a serious attempt to murder him, possibly with a stolen weapon.