With me, it wasn't the world ending so much as the fact that I realized I will someday grow old. When I was a kid, every time I thought about Armaggedon, it didn't sound good to me at all. In fact, it made me very depressed. I had a lot of friends and close family who were not JW's and the thought of them being destroyed brought me to tears.
Growing old, on the other hand, was something I had never even thought of. Not only that, but losing people I love to old age. That just wasn't supposed to happen! I left the religion about 10 years ago, but I still have problems with this at times.