I have said before that pissing is actually a pleasant sensation. Lots of guys describe the sense of relief if they have been holding it a while but I mean the actual sensation of it flowing through the urethra and out of our cocks.
On the other hand, I have noticed that the sensation is much stronger during the first piss after having cum so maybe some of the sensation comes from the prostate or the internal (upper) urethral sphincter which needs a little time to get back to normal after ejaculation having taken it over.
The same may be true of pissing with a hardon. I have always been able to do that, unless I am really close to cumming, the only problem is aiming. But, as we get close to cumming, the internal urethral sphincter is forced closed which is why we can't piss and cum at the same time. I don't think that is a sudden thing in the way that the point of no return is, so probably as we get close to the edge of cumming it is starting to close and, maybe, partially closed means more sensation as the piss flows through it.