why do I always have dreams about the end of the world?

Had another end of the world dream this morning. It was real-like, so vivid and fucking scary as all hell. So. I guess it will be water that kills us all. Better than fire I guess.No I don't know what's gonna happen, when and if anything at all. The idea I have is that us humans will do ourselves in. Run out of supplies, water, food, gas, electric. The ice caps will melt, loads of animals, sea life and plants will become no more. The sun will make it too hot for any humans to survive and so on. That's how I think the end of the world would be. Scary thought isn't it?
Anyhow- the dream seemed so real-I woke up with my heart beating so fast! I know the dream was really long, but only remember parts of it. The first thing I remember is standing on some old man's back porch. This old man I saw on the weather channel a few days ago. He ws a sailor that survived Katrina. I was watching storms stories. Haha. So there we are on the back porch, shoulder level in water. I was standing there wondering why he was sitting at a table across from another gentleman, having a drink with his legs crossed. I was like "hellllo, flood!" I guess this sort of thing happened all the time? The water level dropped quickly before our eyes & everything was back to normal. How weird. Then the next part I remember standing w/ some other guys on a upper level somewhere, looking down at the freeway. I don't know who these guys were. Out of nowhere, these big 18 wheeler trucks come barreling down the freeway. With all their heavy loads still attached. We said "god, those idiots always drive too fast, wonder why their driving so fast?" Then we see all these cars racing down the freeway, some packed with families and supplies. We then really wondered what they were trying to get away from. All of a sudden I am on a train with about 80 people. We were going pretty slow, then the driver stepped on the gas and we were flying, going the fastest the train could go. It wasn't going fast enough. All of a sudden a few of us see this truck. It was like the van on Scooby doo. About 6 of us got into the van and took off. Before getting off the train we saw a huge humongous tidal wave, just a wall of water coming over the trees, as high as the sky and even taking the clouds with them. That's how high this wall of water was! Sometimes I can remember who I was with, others I can't. I think Ursula was there for a second & told me to stop worrying about leaving my shoes behind. Haha. I kid you not! But they are my fav. flip flops that I wear almost everyday, I love them, we have a bond! We do!
So, we get in this van, there's about 6 people and we speed away, leaving my fav. flip flops on the ground. I cried and reached out "wait, my flip flops!" I was so upset. They are like what linus' blanket is to him. You know. So I try to get over it because the end of the world is coming you know. So, we are trying to outbeat mother nature and we are all screaming and yelling "Go Go". The wave is coming closer and closer and the roar was eerie. I wish I could remember who else was in the van w/ me. Damn it. I remember foam from the wave being everywhere. The water came as close to our back tire!!! And we made it out alive!!! The sad thing was, we were the only survivers. I almost forgot- there was this cute cat that was on the train tracks that was panicing and crying out for help. I tried to snatch him up, but couldn't reach him. When we got in the van I asked the driver if we could go back and get him. We had no time. The wall of water was just over our heads. I remember crying for the cat. And in the end wondering if it made it. We were all surprised we out ran the wave, we were all in shock. Relieved, happy, sad, exhausted, spent and crying. We all just sat on the ground crying hard. It was so weird. So vivid. I wish I could remember more. There was more to the end. Hopefully I'll remember later. I'll come back. The intire world was flooded. Even the North pole. You couldn't see any buildings anymore. No Impire State building, No stature of Liberty, nothing in Paris either. It was one big under water city. One big vast, never ending Sea.