1. He won't let you walk away
You might be upset, and want to walk away from the relationship or a fight that the two of your are having. But instead of letting you storm out that door, he will try to calm you down and hold onto you. For him, it is about making sure that you and the relationship are okay. This is actually a sweet gesture and shows that he is scared to lose you, and cares about making things work versus letting you walk away upset and hurt.
2. He puts up with your attitude
You can have a bad day, or just be PMSing, but instead of giving you attitude back or getting upset, he understands you and tries to brighten your day and make things better. It is about putting you first versus his frustration at your behavior. During a time when a girl is upset, she needs to be comforted, and a guy that can understand that, is a keeper.
3. He apologizes
A guy that will apologize when he hurts your feelings, or is late for dinner on his own free volition without you having to nag, knows your value. He will apologize because he wants to make sure that you're happy. This even means seeing things from your point of view and recognizing that your views may not match, but compromise is what it's about.