Lestrade and John start going to the pub to watch rugby/football because it’s a friendship of convenience – they each have someone else who understands what it’s like to have to deal with Sherlock – but over the course of a few months, they start opening up to each other and eventually become great friends who are completely comfortable with each other whether it be drinking and having fun, or talking about past relationships, etc. Sherlock is no longer their only common interest.