This is Louis and Harry when Harry sings the first chorus:
Liam and Niall seems to notice L and H's little serenading sesh, and starts serenading Zayn, who immediately notices and plays along:
Then they turn around to do the same to Harry and Louis who just...
Haz and Lou in their own world.
After Harry finally notices Niall and Liams attempt to serenade them, they sort of already gave up, and then L and H just goes back to