Necessity

 is the 3rd chapter of the 4th volume and the 16th chapter overall of the Attack on Titan manga, written and illustrated by Hajime Isayama.

Summary
Eren is still having trouble balancing in his training for 3D Maneuver Gear and tries to get tips from Mikasa and Armin. However, it doesn't go well and he hits his head on the ground knocking himself out. After some remarks from other trainees, they go to the cafeteria to eat and so Eren can recuperate. Mikasa says that Eren may not be cut out for being a soldier and suggests that he could get another job. This angers him and he reminds her of what happened when the Colossal Titan attacked. Angry, Eren leaves the table. Mikasa tells him that if he were to quit, she would follow.

Back at the barracks, Eren is asking people for advice. After a few dead ends, he's pointed to Reiner and Bertolt. While they don't give advice, knowing where they're from, they do ask Eren and Armin why they are joining the military as they know what Titans can do. They both say that they couldn't just sit around and do nothing. Bertolt tell them where they are from and how they got here. After the Colossal Titan attacked the wall, the Titans got to their village before the warning did. They barely managed to get to their horses in time to make it to safety. He notices that Armin and Eren aren't like the others. They just joined out of ignorance and because of expectations. Bertolt says that he joined the military for the chance to join the Military Police, otherwise he might quit altogether. Eren comforts him, adding that after what happened to him, no one could blame him for wanting that. Eren admits his fear as well, but says that the hate he has for them is greater than his fear. After swapping stories, Reiner tells Eren to check his belt and to make sure it's adjusted and that tomorrow it should work out.

The next morning, Eren is starting his test. The instructor tells him that this is the absolute lowest requirement of a soldier and if he can't do it he can't be a soldier. He tries it again, initially keeping his balance but falling over after a few seconds. The instructor then tells a nearby soldier to exchange equipment with Eren. With the new belt, Eren keeps his balance perfectly. The instructor sees that his belt was damaged. Eren, with a triumphant look on his face looks at Mikasa. She comes to the sad realization that he's glad that he won't have to depend on her. The instructor is impressed that Eren was able to keep his balance, even if for a short time with the damaged gear, and thinks to himself that Grisha would be proud of his son.

Trivia

 * In the chapter 71, Bystander, it is revealed that Keith broke Eren's belt, so that he could not join the Survey Corps and keep his mother's wish.