Almost anything can be described as a reaction to a situation. Right down to Marten's reaction to "I don't have any soup" or Faye's reaction to "Sven is good-looking and I am horny". The difference is that there are passive reactions and active reactions.
I do actually grant that for the vast majority of the strip Marten's been giving us passive responses. He lets Faye move in when she needs a place to stay, then he lets Faye turn him down after The Talk, then he accepts Dora's advances when she kisses him, he does nothing but say "Aw shucks" when he loses his job as Office Bitch, he accepts the position at the library when he wins it by accident - nobody else applied and he passed Tai's secret test without knowing it was a test at all. All of those and many others are passive responses.
But when things started to go sour with Dora, Marten found something he considered worth fighting for. He actually made an effort to keep the relationship going, and fought to stay with her. When they finally broke up, he was bitter about it and not just passively accepting. And since then, well, there's all the examples listed.
Again, I'm not saying that he's gone out and changed the world with these small things. What I am saying is that he's become a far more assertive person than when the strip started, and his character has evolved. And that in the last few hundred strips, it's become more obvious than ever.