Friendly, but not friends

My son doesn't like somebody. It's kind of sad. He's not even three. To be fair, the person he doesn't like is a pretty annoying kid. I tried to explain that he doesn't have to be friends with everybody, but he has to be friendly. For his age that meant, you have to be nice to them.
All the time I'll talk to kids who are really nice, but they feel bad because they don't want to necessarily be friends with some of the kids in our youth group. They feel bad because they can only invite so many friends to their party, and they don't want to waste an invitation.
I just tell them that just because they are both Christians, doesn't mean that they are going to be friends. There are lots of other things that make friendships work. So I encourage them to be friendly, but not necessarily friends. JM