This is so well built-up, from small children growing up too fast, to childish pranks gone a little wrong and then to grown-up love and longing. The specificity of the speech patterns for all the characters really rang true, and I do have to say I love being in a fandom where the words

'Fine, I'm fine.' Remus catches Sirius's eye. 'Thank you for, er. Thank you for punching me in the neck.'

can be written or uttered with complete rhyme and reason.