The Last Summer of the Death Warriors by Francisco X. Stork

From the Hardcover:
DEATH
It surrounds Pancho. His father, in an accident. His sister, murdered. His own plans to trace her killer. And D.Q. - a guy Pancho's age who's dying of cancer.
  That is, if he'll ever shut up.
LOVE
D.Q. is writing the Death Warrior Manifesto, a guide to living out his last days fully. He needs just one more thing: the love of the beautiful Marisol. But as Pancho tracks down his sister's murderer, he finds himself falling for Marisol as well...
FAITH
And choices that seemed right and straightforward become tender, tentative, real. While D.Q. faces his own crisis of doubt, Pancho is inexorably drawn to a decision: to revenge his sister and her death, or to embrace the way of the Death Warrior and choose life.

"What if everyone is given a task we're to work on and if possible complete while we're living on this earth? What if before we are born and assigned bodies, all our souls stand in a line in heaven, and as we get ready to come down, an angel gives us a little slip of paper that says what we have to do? Only the message is written in a language that we forget how to speak as soon as we get down here. But the message on that little piece of paper is still deep inside us, and our job is to remember, to recollect it, and then go about doing what it says."

This was a little slow to start, but once you get going it's magnificent. I didn't feel the characters in my soul, but I did feel the entire story. Who hasn't wondered at their purpose in life?
   Pancho is determined not to let D.Q. get to him, he doesn't want to care and he doesn't want anyone to think that he might. As he tracks his sister's killer he knows that he will end up in prison for it. He feels like he just needs to do this one thing before his life is forfeit.
   D.Q. is determined to live the rest of his life on his terms in spite of his mother's forceful will to cure his cancer. D.Q. has chosen Pancho to be with him while he goes through the treatments his mother has insisted he try. All the time D.Q. tries to show Pancho the ways of a Death Warrior, to make Pancho appreciate the life he has left.
   This book should be read by everyone. It's a story about learning how to really live and enjoy life regardless of your circumstances. I would particularly recommend it if you loved The Fault In Our Stars by John Green. I know that book is widely loved, but this book has some similar themes and it doesn't crush your soul. This is not a love story, it is a tale of a friendship.