Death wish

What makes people lose the will to live?
Sickness, depression, frustration, stress?
I don't know what it is, but I do know that once you've lost it, it's hard to get it back.
I've seen it happen to my grandpa, who after forgetting directions, how to cook, the day of the week, the pills he took, and even me, refused to eat anymore. We took turns, forcing one, maybe two spoonfuls of mushed-up food into his mouth before he'd shake his head and clamp his teeth shut. The doctors told us his body was shutting down, and he was only listening to what it told him.
He died soon after.
Now I'm afraid my grandma is doing the same thing.
When my parents went to visit Tuesday, they found her on the couch, ready to die.
"This is it," she told my mom.
Inside her bedroom, she had written a note to my dad with instructions about what to do with her body. And she refuses to eat.
But my grandma isn't sick. She doesn't have Alzheimer's like gramps did. Her back injury has progressively gotten worse during the years and she refuses to see a doctor for it. She's eating pain killers instead of food. But she's a relatively healthy 83-year-old.
She's not dying from any natural cause.
I'm just afraid that she'll make herself.