Why does my fiance want to keep going back to a negative friend?

I'm sorry, but Jared sure isn't acting like he's anywhere near 30. (Neither is Darren, of course, but you're not marrying him.) Tell him you're not going to control who he hangs out with, because you know you don't have that power. But you are genuinely concerned that Darren is going to try and tempt him back into old habits, and you sincerely hope that he will bear in mind how that makes you feel. Additionally, point out that Darren is not the only guy on earth, and you are happy to help or encourage him in finding guys to socialize with who like and respect the person he's worked so hard to become - the person you love - instead of insulting him for it. If it turns out Jared's having conflicted feelings about being that person and the renewed interest in Darren is just a symptom of that, you may have some tough decisions ahead of you.