As much as Blaine loves staring at his boyfriend's ass, it's not always practical. Kurt does have to sit down sometimes. Plus, if Blaine only stared at Kurt's ass, he would miss out on everything that happens when they're facing each other, when their lips are within touching distance.
That's how Blaine discovers his love for Kurt's hair. It started as a place to put his hands when they made out and Kurt's other assets were unavailable. Now, it's one of his favorite things about his boyfriend.
It's become magnetic. Kurt's cool lips sucking on Blaine's, and suddenly his hands had a mind of their own.
His fingers almost always start at the base of Kurt's neck, scratching just above where Kurt's hair trails off. The second Kurt releases those sinful whimpers, Blaine's hands move up, kneading among the thicker hair at the crown of Kurt's head.
As soon as Kurt lies back against the bed, Blaine's hands shift to the longer hair at the top of his boyfriend's head. With every tug, Kurt moans into their kiss, opening his mouth just a bit wider for Blaine.
When they're in public and he can see the few hairs Kurt didn't manage to fix. Blaine smiles knowing they're like that because of him, knowing that the second they're alone, Blaine's hands are going to find a way to coax all of Kurt's hair out of place again.