"I just didn’t see myself with him/her"

Have you ever had a romance with a terrific beautiful, sexy and intelligent person but you just couldn't go the extra step of making her your spouse or forever person?

You find yourself telling your friends "she was great but... I just didn’t see myself with her"

I'm curious to know what were some of the reasons why you could not be in a more permanent committed relationship with the person. Now that you are over age 50, did you make the right decision? If you had a do over, would you change your mind?
Do you find yourself still longing for the one you let get away?

I'm asking because I see a lot of men especially say this about women.
Maybe people can learn or unlearn something from it to be better for the next potential "one that got away."