Sonny understands that Graffiti Pete’s tough guy persona is half an act. You can’t grow up in the barrio without getting a thick skin but, Pete is an artist at heart and while he may seem a little gruff and thuggish on the outside, he’s really quite sensitive––not that he’d ever admit it.
Graffiti Pete understands that Sonny would be a completely different person if he didn’t have Usnavi; without him, he wouldn't go to school and he'd have no place to go when Usnavi wasn't there. Graffiti Pete can tell that Sonny knows this, but doesn't know how to say thank you. So, instead he does things like attempt to guard the store during a riot.
As Pete helps Sonny with one of his murals, their fingers touch. In that instant, they both understand, even if they'd rather not.