On royal weddings -

Actually, having an alliance tends to lessen the chance they'll let you marry their princesses, not increase it. If some 'old friend' of yours doesn't feel like coughing up a princess, make friends with their enemies, and watch how they come running with wedding bells ringing in the background.

Consider it from their point of view? Do you want to give a princess to someone who's already a friend, or save her to give to make some new friends? It's not like you're going to become more allied with em. So, they need a little...incentive.