Maybe you're not dense, maybe you're just such a genius that pigeonhole principle proofs seem naturally obvious and tautologically true to you? :-)

I found this enlightening (particularly "Sketch of a Proof"), though I also admit that it seemed fairly straightforward, with elegance borne of simplicity rather than of brilliance or cleverness: https://en.m.wikipedia.org/wiki/Theorem_on_friends_and_stran...