Add Isabella into that particular party banter too.....it's good! lol
I just don't think Fenris understands his own emotions all that well. When that scene happens, to me, it's less about the actual, individual memories and more like an absolute emotional overload and he doesn't know how to or can't cope with it. He thinks it easier to run away than to try and understand or accept what he is feeling. My Hawkes are all patient if nothing else and wait for him to gather himself......totally worth it!! 
My Hawkes may also make him slightly insecure when they visit the Gallows and he has to stand awkwardly and watch Hawke drool over Cullen 
Ooooh yeaa I remember that Isabela part! A friend *did* Isabela after left by Fenris, then she'll make the BEST comment. Apparently the best combination is that
1. You do with Anders first, then tell him it's only one night fling
2. Do with Fenris and he'll leave you
3. Then, do with Isabela
That will give the best party banter ever. I love DA2, being the only one in DA series that you can actually *do it* with all 4 LIs (not including Seb).
I'm a huuuge Fenris fans, so everytime I always wait for him 
Also Khalid from Baldur's Gate, he wasn't the most courageous Warrior either.
I didn't play other games from Bioware except the DA series. But sometimes being whiny and showing signs on not being courageous does not convey with the deepest part of the character. Remember Neville Longbottom from Harry Potter? That whiny and scared & bullied & not courageous little boy on the first book turns out to be one of the best and the bravest at the end. (He's my favourite character actually, even more than Harry & Ron)