The brain automatically sends surging pulses of adrenalin to the nerve endings via neurotransmitters which have lain dormant, provoking memories of similar experiences. As your fear receptors are bombarded with warning signals, drawing back now is futile.
Reflexes now act only to be overridden by the driving cravings within. You know the agony is worth it when you really want the butter to melt while being spread on your toast.