He once was the younger one in the relationship, he also was once the "stable" one in the relationship.
He learned how to slow down and what he wants in a relationship after the two.
This led to James being very protective when he started dating Diane.

I'm throwing it out here. Diane is not James's first dating experience. There are two of his exes that actually shaped James to be who he is in his relationship with Diane. The James we know in canon!