SeaDonkey - that bugs me too!
I think guys are often rough & I've seen NBA players pat eachother's rear ends, & it doesn't seem to bother them. Sometimes it could be like, "Good job!"
I've also been patted REALLY hard - almost like they were hitting & that upset me.
But I think usually, people have good intentions. My husband used to pat me on the shoulder - & I think he meant, "Don't worry, you're fine." To me, it came across as I'm a pittied pet, so I told him & he doesn't do it anymore. I imagine it depends on the context & relationship.