Us humans tend to rethink the way we live when confronted with our own mortality. As Box said, you see this in people who suddenly don't have long to live. I saw it in a 13-year old boy I knew some years ago who contracted leukemia, went into remission, then it came back. He made his peace with the world and with God and accepted his curtailed lifetime. There were over 600 people at his funeral - and that church has never had a service so big before or since.
Even little things can do it: I've never been someone who will try anything, but events in the last 18 months have changed me a little and now there *are* things I'd try - you've heard about the kilt, for instance. Having loved and lost, I know I'm definitely living more in the moment now.
And another thing that may help - and I'm not belittling your real feelings - but the way our memory works is that it prefers to remember statistical anomolies. I heard recently of a couple of studies related to how people remember things like gambling wins and losses. Basically the importance we attach to individual events is important in remembering aggregates of them. That's very likely the reason why you feel worried about your peers dying. It's the same effect when it seemed everyone around you was getting married or having children and you weren't: human memory remmebers this and makes it seem more significant than it really is.
Wade.