Deliciously-Demonic wrote...
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Part of me wonders if Fenris actually lied about his memories returning when he...y'know...with Hawke. Considering how spur-of-the-moment it actually was, maybe when everything was all said and done, he freaked out, maybe thinking he was just trading one master for another, or something akin to that. Mostly that face he makes when he leaves.
To quote one of my favorite YouTube comments, "Only Fenris could pull the ultimate booty-call, and walk away looking like *he's* the one that just got used."
Maybe he thinks he was used by Hawke. Maybe he thinks he should be angry, but his feelings for Hawke get in the way of that and confuse him. I dunno. Food for thought. 
I've wondered similarly, although I do tend to think it was memories that bothered Fenris that night. Mind you, it's just what makes sense in my own head and not neccessarily anything overtly described in the game. But I think it terrified him, to love someone, when Danarius was still
out there, somewhere, looming like a dark cloud over his whole future. I believe Fenris simply had to overcome his fear of Danarius' coming along to control or otherwise manipulate him into destroying Hawke before he could truly appreciate the idea of Hawke and him together like that.
I think, anyway, it's memories of the Fog Warriors incident that drives Fenris away from Hawke that night. I honestly believe there was nothing that horrified Fenris more than the fear he might turn on Hawke the way he had turned on the Fog Warriors, that if Danarius came along and ordered him to hurt Hawke would he really be able to refuse. What if Danarius had some means of controlling him beyond the simple "slave mentality", what if his magic could work to compel Fenris towards such an abhorrent act, what if Danarius could make him forget all over again...I think the "what if" possibilities drove Fenris mad as he lay there next to Hawke and finally drove him away.
He does tell Hawke later on, "I thought it better if you hated me." To me, that line throws the entire "All my memories came back then disappeared again" story into huge question. He deliberately drove Hawke away. Which plays into the protectiveness that's so much a part of Fenris, too. The man's entire life history, what he recalls of it, has been as a bodyguard. Fenris protects, that's what he does. If he fixated on protection of Hawke, keeping her at a distance from him would be paramount until he could absolutely sure of himself and Danarius' hold on him. Especially in light of the events in Bitter Pill, watching Hawke rush into those caverns alongside him, to battle the myriad threats there, up to and including Hadriana and her shades and demons.
In my head canon, Fenris fell asleep next to Hawke and had a horrible nightmare about the Fog Warriors. Only it was Hawke rather than the Warriors he ended up destroying. Waking up, he dressed, gave Hawke his sorry excuse of a tale to "make her hate him", then ran away. As I see it, the events that followed, with Hawke's mother dying and then the fight with the Arishok, where Hawke gets a sword crammed through her middle, changed everything for Fenris. I'm not a big "they didn't do anything for 3 years afterwards" fan. I think there was "something" going on between them, but it's just not articulated or openly acknowledged by much of anyone, not even and maybe especially the two of them.
By the time Fenris confronted Danarius, though, the chances of him causing even a niggling bit of harm to Hawke was nil; so when Danarius mocks Hawke and Fenris' relationship with her, Fenris loses it, he glows madly. The look of shock on Danarius' face is perfect -- he expected Fenris to do as he was told, just as Fenris had feared. He never realized how far from that Fenris had grown.
That's just my own thinking going on, though.