Eren is not your ordinary high school student. At least, one would hope that your "ordinary" high school student doesn't have anger issues out the wazoo like this kid has. After he gets in one fight too many and faces expulsion, the school cuts him a deal of last resort: put his time toward a community service program, or hit the road. Eren has some degree of sense about him, so he agrees to this deal. The program he's enrolled into puts high schoolers to work in the homes of elderly and disabled people who don't have any family or friends, helping them with housekeeping, errands and suchlike. No big deal. He expects to end up cleaning house for some sweet little elderly lady with a lot of cats, or at worst a crotchety grandpa.
He doesn't expect to end up with thirty-something, 5'3", military vet Levi, who sets him to work literally scrubbing his entire home top to bottom and doesn't speak a word to him when he's not cussing him out; who he's pretty sure could beat the tar out of him with his remaining arm tied behind his back; who's transparently nursing a shitton of trauma from the car bombing that took, not just his arm, but the lives of his entire squad (oh God I went there); who, despite his better judgment, Eren finds himself oddly invested in the well-being of.
Because, sometimes, a person just needs somebody. And Levi has nobody left in the world but him.