why is it that Halo people can regain health

Me and my brother love playing Halo on co-op and you do not regain your health without using a health pack,your shield does recharge,however,and that is the blue bar that you see under your health bar,and as long as you avoid getting shot,your energy shield will recharge, and there will be times in which the shield will make all the difference between getting killed and not getting killed.

My Fiance and His brother enjoy playing Halo on Co-Op. I'm not sure how it goes along with the story line but I notice that they will shoot one another if they mess up on the game, and they do this so that they can go back to their last save point. I'm not sure if it's any different on single player. I hope that you figure out your answer to this!