If you and I were friends, and you threw me a birthday party every year, and I never returned the favor, probably you would eventually stop throwing me birthday parties. Why would you continue to feel duty bound to throw me a birthday party if I wasn't willing to do the same? (or at least something equivalent)

That's a reasonable expectation of reciprocity and it's not necessarily the same thing as being transactional. Most people have that expectation, to be treated as they treat others.

Charity is something different though. It's when you recognize someone's hardship and feel obligated to relieve them of some of it.

In the absence of hardship, most people tend to be willing to give what we're getting.