The Atlantic has a short article explaining the pleasures of Japanese onsen which is right on the money:
Why is all this fun? Japanese people sometimes explain the attraction with mumbo-jumbo about how onsen provide a spiritual experience. Personally, I think there’s something in the lizard part of our brain that really likes sitting around in hot water with no clothes on in a beautiful mountain setting. Also to be taken into account is that after the onsen, you can put on your yukata (a bathrobe-like garment provided by every ryokan) and eat wonderful food—the culinary quality at Japanese inns is amazingly high.As the article mentions (and has photos of) the Yakuza at an onsen, I should mention that I have never seen a tattooed Yakuza in an onsen or sento. (I understand that they are not welcome at the great majority of such establishments.) I did read in an old edition of the Lonely Planet guide to Japan that a member of the Yakuza may be quite happy if you show an interest in his tattoos. I thought this odd advice: generally speaking, isn't it more prudent to avoid engaging gangsters in conversation about mutual interests?
Speaking of tattoos: God I've seen some awful examples on women at the shopping centre lately. What are they thinking?