It's "Love."

Jeanette: Ted, listen to me. You are being crazy. 

Ted Mosby: I am being crazy?!

Jeanette: Yes! How long have you been hung up on Robin? Eight years?! and you are still killing yourself to fetch dumb little trinkets for her. That's crazy! That's more than crazy. I don't think there's a word for what that is!

Ted Mosby: Actually, there is a word for that. It's "love." I am in love with her, okay? 

Ted Mosby: If you are looking for the word that means caring about someone beyond all rationality and wanting them to have everything they want, no matter how much it destroys you, it's love!

Ted Mosby: And when you love someone, you just, you... don't stop, ever. Even when people roll their eyes or call you crazy. Even then, especially then!

Ted Mosby: You just don't give up! because if I could give up... if I could just, you know, take the whole world's advice and move on and find someone else, that wouldn't be love. That would be some other disposable thing that's not worth fighting for.