Why does it hurt when guys get hit in the crotch, but they beat and slap their penis&balls?

When watching porn, countless of times I've seen guys swing their penis against their shower door, punch/squeeze their balls as they stroke themselves, and what not and I just thought....wouldn't that hurt? It brings them pleasure obviously but any time I've accidentally hit a guy in the balls, they can't get up for several minutes. What's the difference? Why does one bring pain and the other bring pleasure?