But you can't have your cake and eat it...
I think Marten has a mix of positives and negatives in his life, and sure - you can always do more to increase the impact of the positives!
But I think getting rid of the negatives is often much more difficult for people, and that's Marten's issue.
I personally believe that Marten's methods of interacting with some of his closest friends is one of his biggest negatives. Because he's generally quite meek, people project his opinion onto him when they know him well and when he deviates from that, he's wrong.
A great example of some of Marten asserting himself in a positive light would be a lot of his interactions with Hanners, helping her to be less OCD and more social, the drum kit thing, etc.
A great example of Marten being stomped on for deviating from being a door mat would be the starting scenes of the breakup with Dora (specifically the "Don't look at my porn.. you looked at my porn! I told you not to!")
Edit: I guess my point is that Marten does try and be assertive at times, and often the reaction that follows (positive or negative) is pretty much down to who he's being assertive towards. Some of his friends definitely seem to have peg holed Marten, and don't react particularly well when he fails to follow through on their expectations.