When we like someone, we find them more beautiful. If someone is nice, kind, generous, charismatic, it's easy to find them physically attractive as well, or at least concentrate in whatever beautiful physical aspects they have. Beauty is so subjective anyway, the same face can be either horrible or beautiful depending on how you look at it (and who is a better example of that that Spanish actress Rossy de Palma?).
Smiles, when real, also make people look prettier. Whether this is because they make them look nicer and more likable, or whether most faces actually benefit from a smile, I'm not sure. But I know that babies whom I once thought ugly, suddenly become beautiful when they start to smile.