Why do people get grouchy when they try to loose weight?

I help people get skinny, I'm 14: male 65 inches and 140 pounds. I'm helping my cousin get skinny. He is 67 inches 230 pounds he is 12 year old male. He did good on his firs tday BUT! In the afternoon I made him a tuna salad. He didn't like it but I told him no pain no game. He siad he liked carrot (he was probably trying to sound healthy) but I saw him eating the pecies of carrots slow. I tell him and teach him about nutrients while he is eating his tuna salad, but he looks like he's in a grouchy mood... why? Is it cuz he's not eating real food? Well then his dad ( my uncle ) tells him who turned off the tv at the room. And he talked back to him and uncle said "Damn don't be mad just cuz ur in a diet and eating salad." My cousin is usually happy and jolly. But maybe its the salad lol. We go jogging too.