I can tell when the feathers are in the wrong places. [Which you'd think wouldn't bother him, considering his general attitudes toward messiness, but it does.] And water soaks in when I don't fix 'em. Then I drip all over the house after I take a shower and you get mad.

[Technically it's an oil redistribution thing that waterproofs the wings, amongst other things, but Sans has absolutely not done any research into bird wings and how they work.]