Unless a mentor taught him values. Unless a catastrophe changed the course of his life. I build a background step by step, considering future story events and the way I want my the characters to react to them. Motivation propels conflict which makes your romance strong. That motivation springs from your character's background.
Some writers use a character chart to really get to know their characters.
Philosophy of life. From the simple to the complex. You can be as detailed as you need to be. Once you have the first part of your couple, you work on the second half. Mariah clash often.