Why is it that some friends grab my butt when my boyfriend's not around?

A friend of his did it once, another friend of his did it as well on another occasion and yesterday, my own friend did it after I went for a hug. On that last occasion, we were both drunk and it was plain funny.

Should I assume that I have a nice butt or is it just what guys go for when they want to feel a girl up?
Why is it that some friends grab my butt when my boyfriend's not around?
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