It is currently unknown how a grudge is formed, but a dwarf that talks to another that he/she has a grudge with will generate an unhappy thought ("He was forced to talk to somebody annoying lately."). This will happen more likely if you only have one meeting spot or many jobless dwarves.
It is more likely to see a grudge form between dwarves with opposite ideals and/or personalities. A recent in-game example would be between dwarf Mistem Okoluzul who formed a grudge with dwarf Adilina Kubukthiz. In examining their respective "Thoughts and Preferences" specifics, we find an obvious clash:
- Adilina Kubukthiz: "She is candid and sincere in dealings with others. She finds helping others very rewarding."
- Mistem Okoluzul: "He is often sad and dejected. He does not go out of his way to help others."
A personality clash is not necessarily a requirement to the forming of a grudge. Likewise, grudges are not always going to be mutual.Verify
When the object of a grudge dies, it causes a thought in the mind of the grudge holder, but it will be a negative one still.