Why Do Horses Roll?

A know a lot of people probably know this answer but I shall post about this anyway.

There are a whole lot of reasons a horse will roll, such as they do it for the pleasure of a good scratch on the ground or you may have just washed them and they are trying to help out by rolling in a muddy patch.
Other reasons a horse may roll is because its having abdominal pain like colic and they roll to try to get comfortable.
Horses without winter rugs, roll because when the mud sticks onto their coat, it traps the warmth in.
Some horses even roll because the mud acts as a sunscreen and some what prevents the sun from burning tender places. They also roll to stop insects from annoying them, the dirt acts as a bug shield.

If a horse rolls with a rider or tack on, this becomes dangerous because the horse could crush the rider and the tack. And lets not forget.. Horses aren't light.
A reason horses sometimes do this is because they tack is hurting or pinching them and making them uncomfortable, so they try to roll the pain away.

There are so many other reasons like the are stretching, shedding their winter coat, scratching their back or want to roll  just for fun :P

Spec's having a good 'ol roll :P