What do Lily and Zach speak about? What does he promise to do? ch 11

In Chapter Eleven, Zach catches Lily’s eye and eventually holds her hand. Lily tells Zach that she used to be harassed by boys who would put live fish around her neck, forcing Lily either to swim to let the fish live, or to get out of the water and kill the fish. The fish would ultimately die against her. Lily also confronts Zach about how he has changed since being in jail. Zach confesses he gets angry, but he promises Lily that he will not become mean. Zach kisses Lily, and she loves everything about the kiss. Zach resolves to study hard this year and be able to go to college, and Lily assures him that he will succeed. Zach promises Lily that though they cannot be together now, after he becomes something he will come back for her. He gives her a dog tag with his name on it so that she will not forget.

Why is Zach wearing dog tag?