It's an honor and a symbolic gesture, not a binding parental guardian role. Lots of quite young people become godparents (especially when their older siblings have children). Sirius only saw it that way because he was the only one left for Harry besides Petunia. Lupin cares about Harry (and values him through the James connection), and Harry's the one who knocked some sense into him so he actually stayed with his family. I'm sure he was grateful in retrospect.

It wasn't really all that manipulative. Harry was the natural choice. Anyone else would have been flatout weird.

Clio