What is wrong with some people?

I was driving on the highway and since i do the speed limit, i drive on the right side of the highway. Now you always come across those people that love to go 30 miles under the speed limit (not 30 miles but you know what i mean) and it gets a little annoying. This not only happened to me but then i just decide to go in the next lane and speed up to the speed limit and get in front of the slow person. Unfortunetly, that person sped up and got in the other lane and got in front of me and started going slow again. Can someone please tell me why we have people like this on the road??