There are lots of ways of answering this question. I'll keep writing them up as they come to me (from experience as well as musings).
I was told of a neat approach at a local high school Christian group: all the seniors were asked for their answer to that question. In one sentence.
A real challenge. Give me 5 minutes or half an hour and I'd be OK. But one sentence?
Let's give it a go:
"The driving factor is the resurrection of Jesus of Nazareth. In his resurrection he showed that he had dealt with the final human frustration and guaranteed his chief claim: following him makes you part of God's family."