i'm like this too. i don't really consider myself "fickle" - it's just that i may react differently to a person at different times based on whatever circumstances - like @Typhon said, i have different emotions for different aspects of people.
i understand people are flawed, and that people don't always get along, and that despite these things, they also have good qualities. i think that is one reason i don't usually feel lasting resentment towards people, because i have a hard time forgetting their good qualities, especially in the long run.
i would rather be friendly and display goodwill when dealing with others - i think i naturally do this. holding grudges doesn't really go along with that.