Why does Pi credit Richard Parker with saving his life?

Richard Parker provided Pi with a reason to live... if only to survive from being killed. Pi couldn't give up because he wouldn't give in, and as a result, he lived to see Richard Parker's retreat into the jungle.

At the edge of the jungle, he stopped. I was certain he would turn my way. He would look at me. He would flatten his ears. He would growl. In some such way, he would conclude our relationship. He did nothing of the sort. He only looked fixedly into the jungle. Then Richard Parker, companion of my torment, awful, fierce thing that kept me alive, moved forward and disappeared forever from my life.