The same reason why they pee in bushes, in alleyways while drunk, and into empty Coke bottles while on long road trips during college: because they have to go, and no one is looking. Also, they’re lazy. If he’s showering with someone, he should ask first. And clean the shower afterwards. If anyone else is using the shower, he should clean it. Actually, it’s safe to say he should clean his shower period. It’s probably filthy.
What, you’ve never done it? Not once? C’mon, fess up. Some women do it too. Well, Madonna at least. She told David Letterman she did it during that infamous interview from like a million years ago. Remember when that was considered controversial? Man, the world was a different back then. The YouTube video I linked to above has a freaking VHS time code for pete’s sake. Madonna also claims urinating on your feet cure’s athlete’s foot, which I don’t believe for a second. Anyway, peeing in the shower. Does it bug you? Tell him to use the toilet like a grown-up. And make him break out the scrubbing bubbles afterwards.