Kid: end of the world. In the late 80s a lot of religious loons would talk up the year 2000 and people would predict the end of the world coming and all. They talked about it in church and it just scared me to death. Now I think about it and thank science and reason for making me an atheist/agnostic type so I don't have to worry about silly unnatural nonsense.
Now: just getting old. I worry about Alzheimer's and dementia and living a hollow existence. Snakes too. Poisonous ones anyway. Snakes would be cool if they had charm and charisma, and could hang out like dogs, but they don't.