One of the things the show did from get go is show why these two people would be friends. Sherlock seemed so utterly flabbergasted/pleased at how John just straight out called him amazing repeatedly and John is so fascinated by the bizarre life Sherlock leads and how brilliant he is. It's wonderfully done and the two leads do an excellent job of showing two people who instantly connect.
Plus, they worked in all of the tropes in a new way which made me giggle.