Can't put my finger on it!

When it comes down to it there isn't just one dream that really stands out to me. I have two dreams that come to mind. The first is stupid dreams where you are trying to get away from someone or something and your running but feels like you are running in place. Then the second dream the reoccurs a lot for me is when you dream that you are falling off of a cliff forever and ever til you finally wake-up.
It always seems that in the dreams where you are trying to run away from something that you get nowhere. As if you were to be running on a treadmill. Or you were to be in a Halloween movie with Mike Myers chasing you. Even though he is not running and you are, he seems to always catch you. I hate to make this next statement but every time I am running away from something in my dream, that something is always a spider. I hate spiders and always seems that is what i am running away from.
Of course next is the classic dream of falling forever. This is a dream that used to occur to me a lot back in the day but not so often anymore. I think the only difference that I had with my dream of falling from other people is that once you hit the ground and wake up in your dream, I actually wake up on the ground next to my bed after falling out. Luckily, my bed is about a foot off the ground so that way it is not that far of a fall.
Really, it just makes me wonder why is it always these two dreams that reoccur for me? Do I keep having these dreams because that is what I continuously think about before I fall asleep? Which that is a sweet fact I happened to come across in high school psychology. Normally what you are thinking about right before you fall asleep you have a higher chance of having some sort of dream about it.