What does it say about a person who cannot agree with you?

I have a really good friend, we've been friends since childhood. And i have noticed that for a while, he just does not seem to agree with me about likes/dislikes and constantly changes his mind. Its really odd and confusing to me. For example, i give him a music cd and he asks me what are the best songs, and i say i prefer the first half of the album and then after listening he says he likes the other half more. Its always stuff like this. Like if i act all excited about a holiday or country i plan on visiting, he will always have something to say to bring it down a notch. Also, if he introduces me to some music or a tv show or whatever, after a while when i get into it and decide that i like it, he suddenly says that its not that good anymore, and there's something better. Can anyone relate to this or make sense of it? Why would someone constantly disagree with you even if it may not be their opinion?