Breatheee...Just Breathe

Why do we forget to breathe while we are working out? I've always wondered this. I remember coaches always saying "don't forget to breathe!!" which seems odd because how could I forget to breathe? But I always do forget in the end. I suddenly find myself while doing push-ups or sit-ups gasping for air because I completely forget to breathe ( I know I'm not alone with this)! I think people hold their breath because while working out they have to keep their stomach muscles tight. It's difficult to tighten stomach muscles and breathe deeply. So maybe that is why? Or maybe it's just a habit that is difficult to break. All I know is I subconsciously hold my breath, and I can't seem to break the habit.