Insomnia

I definitely don't understand it.
Why can't i ever fall asleep? I know way too many people who fall asleep as soon as they go to bed.
Why can't I be like that? It's happened to me maybe once or twice in my life, but only because I was too exhausted to move or think anymore.
I was even like this when I was really little. My parents had to take me for a drive in the car for me to get anywhere close to sleep. Even then, I pretended to sleep most of the time for their sake.
Sometimes it gets really bad, and I get about twenty minutes of sleep each night.
I just don't understand it. I'd rather it was some awful contagious disease. At least then I'd know the cause. My mind just refuses to ever turn off without a fight. Plus, it usually gets worse when I have less stress.
How does that make sense?!