Why do smart people do stupid things?

Don't you ever wonder why the smarter people seem to do the dumbest things. Take for example crazy people when they are driving. You tend to see more "smart" looking people as the most aggressive drivers. I have seen many of them drive recklessly. They seem to swerve in and out of traffic without signaling. Some of those people follow people so close that their headlights disappear in your rear view mirror. When some people end up going to the same place as you are, they seem to want to speed and pass you. Once they do pass you, they end up slowing down so much that it makes anyone angry and want to pass them. Once you try and pass them, they speed up not let you pass.  Everyone hates these types of drivers. There are some people that drive on the same streets day after day and still don't know the speed limit. The thing that aggravates good drivers the most is people that don't step on the accelerator when the speed limit says 40 and those drivers go 30 or 35. It's the most annoying thing ever. When others would try and pass them they would speed up and then when you try to get back into the lane, they slow down. I know that there are some smart people out there that don't use any common sense or common courtesy. There should be a law that if you are a stupid driver, then you have to pass the driving test again. These just some of the things That are most common from what I've seen. What have you seen?