Why are men so insecure about the size if their penis?

HAHAHA i love the last three lines, class. But seriously, its mainly because we men feel diminished by the size shown in porn industries etc. and we also hear that "size matters" so obviously we are going to lie about how large it actually is since its more of a manhood and pride thing than anything else. On the note of the dating website, well lets just say that they're are a lot of perves around the world who are sick like that