This is standard cat behavior.  The likeliest explanation that I have
seen goes back to the way a mother cat treats her young kittens.
When they first are born, she licks their rears to stimulate them to
relieve themselves, and licks up the resulting mess as they do so,
keeping the sleeping area relatively clean.  Once the kittens get old
enough, they instinctively go away from where they sleep to take a
dump.

A domesticated cat treats its human in many ways as if the human were
its mother (for example, meowing when it sees the human).  So, Harvey
is inviting you to give him a bath, starting with his butt.

He doesn't want you to *look* at his butt, he wants you to *smell* it.
Cats do this to each other as sort of a "getting to know you" thing.
He's just being friendly.  Go ahead.  :-P

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Krista

You aren't *really* a furry until you can react just as instinctually as he acts.