ME 1 Reapers:
Our motives are beyond your comprehension because WE are beyond your comprehension. You are nothing and you will die like nothing.
ME2 Reapers:
Our motives are to expand our species and improve yours. Prepare for ascension, mortals.
ME3 Reapers:
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.