Complaints for some can b a way of connecting with people thru a strong emotion. If u r not good at intimacy a shared bond of emotion is a way to make contact with others. Often it's not really about the person. U will find the animosity gets transferred to someone else when they leave.
angry & hate tends to become an obsession like that.
Amazing isn't it? Giving someone (or idea or opposing position) you dislike power over the way you feel, act, and react.
Because they are on the mind. If you have feelings about a person positive or negative and you are reminded of them daily basis it might just take time to stop thinking about that person. maybe something has been left unresolved or without closure, it is very hard to stop thinking about someone who we believe we have been wronged by
It can be jealousy
It can be they don't really hate them but actually like/love them instead
Maybe they secretly liked you/them. It is kinda weird.
Most people haven't the slightest clue who I dislike. Why let them rent space in my head? I agree.