Dora's recent troubles have activated his Big Brother alarms. He's been there for her, he's honestly done his best to help Marten and Dora stay together, and he gave Dora a place to stay without hesitating when the relationship ended. Plus, from what I can see, he's generally been a decent and caring human being around her for the duration (aside from a couple of inappropriate remarks about Ms. Reed.)
Aside from that he was actually a gentleman to Hannelore on their quasi-date, and aside from some mild teasing he was actually pretty nice to Marigold. Plus the whole character arc he's on was kickstarted by Faye causing him to, y'know, grow a conscience.
I've said in other threads that I've always seen Sven as a foil for Marten. Both skinny Indie-rock nerds, they dress alike, they share many interests, they both had the strong connection to Dora and to Faye... but where Marten was quiet and polite and nice, Sven was assertive and womanizing and flippant. He slept with women Marten wouldn't have approached. He's a fairly wealthy musician while Marten's band barely exists. His looks draw attention while Marten blends in with the wallpaper. He's the Anti-Marten, like him in so many ways, but his polar opposite in so many others.
Now, I believe that the path Marten's on is leading him to become more assertive. He's willing to approach people, to try new things. He actually shows emotion rather than just passively reacting to what happens to him. He's developing more of a sense of humour and fleshing out a little more of the world around him. There's many examples of that in the comic ever since he first hooked up with Dora, and even since they've split up.
Sven, I believe, is taking the other path. As Marten becomes more assertive and confident, Sven's becoming more humble. He's paying more attention to the people around him, trying to be less of an asshat, wanting to be a good person instead of the callous dick that ruined his relationship with Faye, had Dora half-hating him for years, and stopped him from making any real meaningful connections in his life.
I don't know if we'll see Sven's entire journey, or where it ends up. But I think that his path mirrors Marten's in a lot of ways, and eventually they'll both find middle ground like this.