Desire to go high forward

Going far from his home, Eren Yeager enters the cadet training corps. There are people who don't understand him. Why Eren is so closed, why Eren is so cold, why Eren tries so hard in training??? Of course, those who are aiming for places in the Military Police already know why Yeager is trying so hard. Eren has no friends, no relatives, he is a loner who wants to make his way up the service.