He tries to talk to Fenris about it and relate to him, and Anders realises that even if he loves Hawke, Hawke hasn’t been through the abuse that Anders and Fenris have. When Fenris dismisses his attempts to talk to him, Anders gets really frustrated and annoyed with him. This continued for years, until Anders eventually had enough and gave off an image ‘hating’ Fenris, and hence approving if Hawke gives him to Danarius. More than anything, Anders just wanted Fenris to accept him as an actual human being, and he was silently begging for someone to talk to that understands the abuse he’s been through. It actually hurt him really deeply that Fenris seemed to disregard him so flippantly, but he covers it up with his supposed hatred for Fenris.