Silly Dog

I was outside one day when I saw my narotic dog Trixie, chasing her own tail. I was wondering why do dogs sometimes do silly stuff like that as well as chasing shadows and such. Is it because they get too excited, or maybe they have an itch on their tail when they chase their tails. Then again, they could be just bored and do stuff like that to kill time. If anyone could shed some light on the subject that would be very helpful.