'upon' is seemingly used when someone or something is literally above and on an object, whether figuratively or literally...I think!<br/><br/>But here are just two examples that do not require 'upon' yet seem more idiomatic with upon instead of the techincally correct preposition 'on.'<br/><br/><i>The letter was looked <b>upon</b> as amateur.</i><br/><br/><i>He was called <b>upon</b> to re-unite the the friendship.</i><br/><br/>Can someone set me straight please?