Well...try putting a ring on someone else. Sometimes it gets slightly stuck at the knuckle. When you are putting a ring on your own finger, you will push your two hands towards each other to get the ring past the knuckle. Well, when you are putting a ring on another person, that pushing of the hands towards each other is not as smooth as when you do it to yourself because your pushing at one angle and the other person is either pushing back at the wrong angle or your pushing the ring over their knuckle is pushing their hand backward, etc.
Anyway, people don't want to look stupid by trying to force the ring over their partner's knuckle. So when they reach that resistance point, they just stop and the partner will pull the ring up the rest of the way.