What kept Pi alive?

If you are referencing The Life of Pi, there are a number of arguments that can be made about how Pi survived. One of the most cogent is his faith. As a student of religion, he makes connections to his spiritual life in his moments of deepest anguish. Further, his religion helps him understand the suffering he is experiencing.
Another possibility is his deep empathy. Because Pi is so attuned to the thoughts, feelings, and experiences of his "animalistic" fellow passengers, he is able to befriend, care for, and even at times control the "tiger," Richard Parker. In no other realistic circumstance could a young boy essentially tame a wild, predatory cat—one who fights a shark and wins, one who is starving and emaciated and ravenous, one who looks at Pi like he is food. But because of Pi's immense capacity for understanding and empathy, he is able to overcome this barrier. His relationship with Richard Parker is what ultimately saves him.