My dad has such a sad life!?

He been in the hospital for 6 years now. In a state of coma. He had brain damage from a heart attack. Ever since he got out of a coma he was paralyzed and could not move. Sometimes he could talk, sometimes he couldn't. Once and a while I visit him and I see he is in so much pain. Sometimes, though, he seems happy and laughing.( barely moving his teeth but you can feel it). For me seeing something like that makes me sad. I Do not want my dad to suffer like this yet be happy.. Sure he is just being positive about it but... day after day he just sits on his bed. Not able to communicate with anyone. I want him to be released from this pain but to do that, he has to be killed by force or starved. Doing that to him now would be cruel. My mom thought that he had a chance to come out of his coma and come back with us. The doctor said he might as well not make it. So save your money and put him to rest. But no.. My mom didn't want that. Everyday I think of him and what is he thinking of. This is interfering with my life and it's not gonna go anywhere. Sure the doctor could put him down but bearing the thinking about it. Letting him be immobilized for 6 years of no good food or communication then all of a suddenly him dying is a cruel way of saying goodbye. I want to be happy. But to see my dad like this.. I cannot. Tell me to move on, I cannot. he was always there for me. So as my mom but to experience this so young is not fair. I am older than 13 but younger than 18 BTW.