What keeps him going? Abraham Maslow called this the need for self-actualization. The need for self-actualization emerges after the basic needs for food, shelter, and friendship have been met. When self-actualization comes to the fore, the other needs are often sacrificed for the sale of this higher one. "The writer must write, the painter must paint, and the musician must play music."
Of course, every theory has its critics. Psychology does not have a "theory of everything," any more than physics does. But however we choose to explain it, most of us would agree agree that Steven Hawking is literally too busy to die. He wants to find out the secret of the Universe! As illustrated by the following two videos, his life provides irrefutible testimony to the fact that you can overcome almost any kind of suffering if you can find something to do with your life that you can believe in!