The fact of the matter is that I probably don't any more than a lot of other people. Granted, I can say that I'm better than SOME people, like Nazis or Serial Killers or Rapists or general low-life scum like the people who beat others up for fun, but I can't really say that I deserve life more than a LOT of other people.
When I was a kid, that REALLY bothered me - immensely in fact. The idea that kids, who were really no different than me, were dying all the damn time elsewhere in the world. Why them and not me?
Well, the real answer is random chance, circumstance and luck. I happen to have been born in to a moderately well-off family that lives in a peaceful, well-developed country. Did I do anything to deserve this? No. But neither did I ask for it. It was simply random chance and circumstance that I have a life as good as the one I've got. That and nothing more.
But even though I didn't ask for it, I have benefited from the actions of others. I still need some reason to justify my existence and the resources I was given - I didn't actually work hard for the life I have now, and I recognize that I have been given a HUUUUUGE start in life due to the wise and clever actions of my ancestors (all of my human ancestors who made all this tech for me to use). I owe a debt to society.
How can I justify it? By using the resources I have to make a difference. If I use the money and security and resources I have to make a difference in the world, I can feel less guilty about having them. If I can give something back to the species that has given so much to me, then I can sleep at night.
It's one of the myriad reasons I went into science and research. Because that way, I can honestly contribute to something that will improve our species and help our understanding of the world and maybe, just maybe contribute to making the world a fairer place. The fact that the world is INCREDIBLY unfair and cruel is obvious to even the most dim witted of us. We can sit on our hands and wail that the world's bad and there's nothing we can do about it.... or we can put our hands together and work towards making the world a fairer place. I firmly believe science is the key to making that happen.