Why do dogs like to follow you into the bathroom?

My dog would stop whatever he’s doing or whomever he’s following, just to be in the bathroom while I’m showering. Is this a common behavior in dogs? Do they find the sound of water comforting? Do they like to sniff the steam from the tap water? Or is it the shampoo scent? I’m perplexed.

So I always crack the bathroom door open, just so he could pop in and plop himself on the clean rug.