Sherlock is WRONG about his job/intelligence being isolating. It's also his choice, in some way, and the show is all about dismantling his idea about not needing anyone.

Yes! Quite. 'Seeing the puzzle' doesn't have to be isolating at all. Quite the opposite in fact - seeing all the connections between things could help him to be a wonderfully empathetic (is that a word?) person if he wanted to. (See the scene where he sends a bottle of champagne over to the newly engaged couple). It's mostly his own choice/his own destructive behaviour that is isolating, not his abilities. Joan takes none of his shit and helps him to accept that other people can be a Good Thing.

I'm not usually that keen on voice-overs either. I think they shouldn't really be necessary...but I love that quote, it's so simple and heartfelt, and it felt right to use it here.