Another old one found in the darkest depths of my old laptop.
James Potter is a protector.
To feel whole, he needs to be able to look after someone. He needs to be in charge of him; he needs to be able to control him, not in an obsessive way, in a... well, a protective way. He needs to be able to guard him, make sure he doesn't get hurt too often. When he falls over, James needs to be able to bandage up his knees. When he's vomiting, James needs to be able to be there with him, rubbing his back, taking him back to bed; when he has nightmares, James needs to be able to comfort him. It's not really all that weird. It's just part of who James is.
Sirius Black needs to be protected.
He's so vulnerable and fragile, and he needs someone to protect him from the people who'll take advantage of that. He needs someone to tell him what to do, where to go, why to do it. He needs this because he finds himself lost and confused so easily, because never in his life has he made his own decisions, and starting now isn't easy. He needs someone who'll stop him getting hurt, who'll clean him up when he falls over, who'll be there when he's upset and comfort him when he has nightmares.
That's not all that weird, either. It's just who Sirius Black is.
And then they found each other- or, being the more outgoing and confident one, James found Sirius-
either way, it ended up with James protecting Sirius. When Sirius gets hurt, James makes him laugh and forget the pain. When Sirius is sick, James tucks him in and stays by his bed all night. When Sirius is scared of something, James makes sure he knows that nothing'll ever get to him.
It's not weird; it's just who they are.