Lestrade doesn't see Mycroft's black town car often, but five times in two weeks is unprecedented and he is not as stupid as Sherlock thinks he is.
Lestrade met John Watson on a case, one that hadn't been interesting enough for his consulting detective. Or: When Gregory Lestrade meets John Watson before Sherlock does. (Firmly a Mystrade fic)
He realised that some officers were able to maintain reasonably healthy relationships and remain on the force, but he had tried that. He was too intense, too passionate, to balance his life to the satisfaction of a partner, and when given the choice between a partner and his job he had chosen his job. Every time.