I always dress well when I fly Business Class (I never fly First). 

Why?  Because if am flying Business Class, I am flying on a friends-and-family coupon from someone who works for the airline, and the airline (for some reason) insists on a dress code for deadheads like me.  (Not _that_ kind of [Deadhead](https://en.wikipedia.org/wiki/Deadhead), the [not-high kind](https://en.wikipedia.org/wiki/Deadheading_%28employee%29).) Slacks, not jeans; button shirt; socks.  Last time I did this, I was flying to a funeral and the standard for the funeral was considerably lower.

If you are paying your own way, you can fly in your boxer-shorts, if you are so inclined.