It's a really nice dynamic between the two characters. The two are a match made in... well, not heaven, but someplace nice. He is good for her in that his devil may care approach to life should rub off some over time, helping her come to terms with her baggage. She is good for him in that he is a challange, something (that if he views is worth the while) he'll have to work for. The real kicker comes from them having already been together physically in addition to the both of them being able to speak on equal footing. Thus, he knows there is something more there than a cheap score. It doesn't hurt that they had a good time in the sack though. If anything, that alone speaks to him in a manner he's familiar with.
I can, however, see the complications of this coupling. Raven could prove to be a source of short term drama. Sven is a reformed-ish manwhore, which is bound to rear it's ugly head at some point and trigger her abandonment issues. Then there's the awkward social dynamic between the two of them with Martin and Dora to take into account. Though, looking at it, it's not all that unusual a scenario. Bawb knows I've seen (and been in) far more complex positions than this.