So why do we think this sexy doc might not have the Hastings' best interest at heart?
His obsession with the Hastings girls: Listen, we get that Melissa (Torrey DeVitto) is smart and pretty, and Spencer (Troian Bellisario) is clearly awesome. However, Wren is a cute, eligible, seemingly very sweet doctor. You'd think he could easily find someone else to help him move on from his old fiancee and her high school-aged sister, and yet he keeps coming back. He helped Melissa with her pregnancy, and he won't let Spencer go. Maybe he's just a stand up guy with a big crush on Spence, but maybe he has an ulterior motive.
He's always in the right place at the right time: Unless Rosewood only has one doctor, there is something very suspicious about the way Wren keeps getting assigned to the Little Liars and the people they know. And the way he just happened to be there after Spence broke up with Toby (Keegan Allen) at the end of the summer season, and just happened to be at the bar where she was supposed to meet Melissa... Well, that's an awful lot of coincidences, is all we're saying.
He has a reason to target Toby: Just because Wren might be "A" doesn't mean he can't also have feelings for Spence (though if he is "A," those feeling are probably more obsessive and creepy than cute). Wren being "A" would explain why "A" seems even more fixated on destroying Spoby than s/he does with messing up the other Liars' love lives.
He's been around since the beginning: Wren has been a part of the story since Day One. He seemed innocuous back then, just a random side story to establish Spencer's character, but then he kept showing up again and a again. Making a side character who has been part of the story all along turn out to be “A” seems like just the kind of twist the writers would love to throw at us.
The biggest problem with this theory is that we can't really think of a good motivation for Wren to be "A," and we're really not sure why he would have been stalking Alison (Sasha Pieterse) back in the day. But who knows. A flashback could explain all of that.