ME 1 Reapers:
Our motives are beyond your comprehension because WE are beyond your comprehension. You are nothing and you will die like nothing.
Our motives are to expand our species and improve yours. Prepare for ascension, mortals.
Our motives are really fucking retarded. We have to destroy everyone to save... everyone? I don't know- seriously man, just roll with it. We have no clue what we're doing either.