This is easy to answer. In many cases, we as men are going to find the dommes we see attractive and in great shape. I basically want to see what their pussy looks like and see how their piss tastes. Also, maybe subconsciously since this activity usually occurs at the end of a session and after they're done pissing, they usually say to jerk off, maybe I associate alot of pleasure with the activity.

Now the interesting thing is..... yeah, I guess pissing on someone or into someone's mouth would be a dominant thing, but in order to do that, the domme would have had to have drank water leading up to the session and basically be uncomfortably holding piss inside herself for much of the session duration to fulfill the client's request. So is that really dominant? I know how uncomfortable I feel holding piss and always feel like I want to get it out of me.