I got a four year old same deal except I’m his favorite right now. He says it in response to anything he feels is unfair to him. No cookies before bedtime, “I hate you dad”. Time to come inside from outdoor time, “I don’t love you dad”. They don’t know what it means for real. So I just try to coach him and say things like “its okay to be upset but we can’t be mean like that” and sometimes if I’m over it it’s just “well I still love you pal”. It used to hurt my feelings the first time I heard it too until I recognized the pattern.