Lets look at it from Steve's perspective.
Steve's major girlfriends (from what we've seen) have consisted of an overly-needy marine student, a pathology assistant who still lived with her literally perfect ex-boyfriend (whom she dumped Steve for, and promptly married him by all accounts), not to mention Tortura,the scarred Russian Villianess who had Steve in what looked like a torture/bondage session, now on the run from the Department. And currently, but not least, we have Cosette, a woman whose clumsy mishaps would put Inspector Clouseau to shame.
Then, we have Steve's father, a man who has been married 4 times, or perhaps three times, even Steve is a little confused about that, which might indicate that his father has a revolving door leading to his bedroom.
So, Steve looks at his best friend, Marten Reed, a guy, who looking at his past should be all kinds of messed up - parents divorced, gay dad with a nightclub, world famous dominatrix for a mother. Yet despite this, Martin is relatively normal. So, imagine Steve's surprise when his normal pal ends up being in a relationship with a hot, former goth, without, to him, many inhibitions, a fiery ass and is a successful buisness woman. Even Steve has told Marten that he wished he could have the same kind of relationship with a girl that Marten had with Dora. Suddenly, Marten is the relationship role-model for Steve, especially to the point where Dora is moving in with Marten.
And, oop, pretty much out of nowhere, to our resident himbo, Dora and Marten have broken up, the status quo has been shaken up, and now, for Steve, seeing what looked like a stable enough relationship collapse like that has given him cause for concern, and now that mental filter is forgotten about as Steve's insecurities have started rising to the surface.
Duuuuuude, Steve is turning into the Dora in his relationship!