SPOILERS AHEAD!
Hee. Let me know what you find in there.
Well, I think Deniz has got to have some massive abandonment issues. I mean, his mom basically said "bored now, you take him," and even Marian occasionally ditched him to take care of his own stuff when Deniz was mid-crisis (chasing Jenny around the woods while Bulle is terrorizing the boys, for one). Plus, I mean, all of Deniz's friends have left Essen (Tim, Nina, Vanessa, even *gag* Dulla)... I can't help but think Deniz must have "I lose everyone I love" bouncing around in his subconscious. I think he's horribly insecure about losing Roman, seeing the way he got so quickly defensive with Marc, and his continued distrust of Marc, and even the very subtle signs of worry he shows whenever the subject of Roman cutting ties with Marc comes up. Whether that's the result of self-esteem issues, or his own experience with (and therefore knowledge about) cheating, or some combination of the two, it's hard to say. It'll be interesting to see, over the next month or so, if it's really obvious what Roman's doing and therefore clear that Deniz is in denial, or if Roman lies really well and Deniz could actually believe all is well. *settles in with popcorn and vodka*
And that's not even getting into all the worry Deniz clearly has thinking he's not good enough/right for Roman... summed up quite nicely during the breakfast threesome. The culture gap, the age gap, there's a lot coming into play that's got to be making Deniz worry; on paper, Marc makes WAY more sense for Roman. But of course, on paper =/= what we really want or need. Still, that's GOT to be scaring the crap out of Deniz and probably is driving him to try to be even more perfect for Roman and... um. Hold on, there's something in my eye.
Ahem. Ok, so, I've heard a lot about Roman's esteem issues and how they're affecting the story, but so far there's only been a few hints about Deniz's. Hopefully they'll get elaborated on more as the stoyline progresses, because I think they're going to be extremely important in explaining why Deniz is so oblivious about RoMarc in the coming weeks.
BTW, my apologies if someone's already said all this elsewhere in the comments, I'm answering this before I go read all the shiny.