Other than mechanical failure ... either drunk or retarded
Well, given the number of pilots who have landed wheels up without any of those reasons, there must be a few other options. Lemme see ... an emergency which was taking up all the pilot's brain space at the time, a non-pilot passenger unexpectedly flying a bugsmasher because the pilot had passed out/was no longer with us, efato where the pilot considered that wheels up was a better option than wheels down, electrical failure, ...
The next one may be yours. Call back sometime when you have enough experience to realise that you aren't infallible