This is very true. I guess the way I see it is he wouldn't be jealous exactly but he might feel like Natasha was with him because he didn't kill her and brought her in and her life changed (although that was mostly her doing of course)... So maybe he would see her with someone else and think its time to let go, let her choose to be with whoever she wants to be with. Especially if there is an age difference that appears to be there in the movie.

Or maybe not. This is why I don't leave prompts, not exactly miss creativity over here. :P