Absolutely true about the two divisions in people.

I had an introverted girlfriend who enjoyed my tradition of inviting couples over for small dinner parties, but if said couple didn't reciprocate the invitation within some arbitrary period, she would claim they were impolite and we shouldn't invite them anymore.

My view was and is: the number of people happy to be invited to a party is lower than the number of people who are happy to host. It's not rudeness, nor even selfishness; it's sometimes just a flavor of introvert behavior.

If this weren't true, you'd be too busy with invites to think of some friends as "they always throw great parties".