Fear and pain. They tie the mind to the body.
Without fear or pain, the mind would drift off into the void, consumed by the ethereal chaos.
Think back to a time you were high. Medicine, drugs, booze or whatever else, just remember an instance when you were high. Felt good, didn't it? But, there was no pain, was there? No fear, right? Barely felt anything, did you?
Fear and pain link the mind to the body. When you hurt yourself, it affects your mood. You become enraged, or disoriented. When you get scared badly enough, your body freezes up, leaving you petrified.
So, next time you smack your head or nearly have a heart attack, don't curse it so quickly. Just remember that negative emotions exist for a reason.