DO NOT READ UNLESS YOU HAVE ALREADY SEEN "THE FOUNTAIN".
My mistake in watching this movie was to think too much. I left "The Fountain" confused. I was looking for complex metaphors where there weren't any. Only after talking about it with my girlfriend did we realize this...
Because his loved one was dying, Jackman's character saw death as a disease which needed to be cured. He put all his energies into keeping his wife alive and in the process, while experimenting on the monkey, discovered the secret to eternal life. His discovery came too late and he became obsessed with the tree he had planted above his wife's grave. He nurtured it and did all he could to keep it alive because it was her new body. He couldn't shake his obsession with defeating death. He then tested his own cure on himself. He lived for one thousand years and kept her tree alive and with him. This was his "cure" for her death. He could not let her go then, in her human form, and cannot let her go now as represented by the tree. Here it gets a little sketchy...my opinion is that he outlived humanity and became the last human being on Earth. At this point he decided he was going to reach that nebula his wife had spoken of, not sure if this was meant to be literal or through some sort of meditation. My impression is that he reached the nebula metaphysically through meditation. The lines on his arms came to represent his long-lived past, like the rings of a tree. Not till the end of his journey did he realize that death is necessary in order to create new life. He learned that death is not a disease or an evil, but an inevitability and a requisite for birth. The book his wife had been writing was trying to teach him this all along. The story mirrored his current inner-conflict. The Spaniard sought the tree of life and became so obsessed with defeating death that when he finally found it, he abused it. He didn't realize that such a creative force must destroy first before it can create. It used the Spaniard's life to create new ones. His wife wanted him to "finish it" so that he could learn this lesson, finally let her go and be at peace. She knew this all along, that is why she was not afraid of dying. She knew that death is just a journey back to birth. So Jackman's character realized this, finished the book, and gave himself to the stars. RIP.
Yes, this movie seems esoteric and pretentious at times. But when you bring it down to a basic level and ignore all the metaphysical stuff and stop looking for complex metaphors, you will find a wonderful, compelling and valuable story of life, death, love, and finding balance.