I've had a suspicion for awhile now that Maren and Dora's relationship is not going to last.
I'm not exactly sure on comic time, but I'd wager a guess that Marten has been dating Dora for well over a year now. Now I know that relationships start with an initial flare that dies down, but their relationship is looking seriously stagnated from my side. Whether it's from hesitation on Marten's part, Dora's paranoia, or a little of both, the pair seem like they are going nowhere, fast.
I believe that Marten's look in the last panel was something more than Dora brushing him off. I think Marten just finally realized that Faye is over him, and that his chance is very much gone. Even though Marten has stayed incredibly faithful to Dora and seemed to have no interest in another girl, he has always had the thought in the back of his head, that at any time he could have Faye. If things didn't work out with Dora, he figured, Faye could always be right there.
But now that's gone, and Marten has to take a hard look at where he is. Alone, in a relationship that is going nowhere fast, suddenly without his subconscious safety net. I don't know about you, but I'd be pretty contemplative there myself.
This is all on Marten's head now: Dora knows nothing about this. Maybe he'll just let the thought rest and leave it at that. Maybe he'll try to advance his own relationship. Maybe he'll panic, and in his haste, ruin it. I don't know, I'm not Jeph and I don't have a complete grasp on the minds of the characters.
All I know is that look Marten is showing us means some changes for him, one way or another.