Deadpool and Harley have a very similar childhood and present too. Both are crazy in love. They know the meaning of love. They are emotionally attached to their love interest. They can do anything for their partners even they can die for them. Like when Joker said her to die she didn't think even for a second and jumped into a toxic tank.
Similarly in Deadpool 2, when Vanessa died, deadpool wanted to die. Even he tried multiple sucide attempts because it was the only way to meet her.
This shows how serious they are for their love. How crazy they are. They faced torture even after they met their love. For example in first movie of deadpool, Wade Wilson only went for experiments so that his disease can be cured. He faced torture many times just to live a healthy life with Vanessa.