Well I think all titans are human controlled, just not consciously. Like Eren lost his control in the last ep, the titans might be in this constant uncontrollable state. I think it requires some mental tweaking (injections Eren's dad gives?) to not only become one but to consciously control it also.

Eren just needs to hurry the F back to his old basement and discover some substantial shit so we may take some educational guesses as to how the titans came about and for what purpose.