Desire:

Kelly McGonigal, The Willpower Instinct:

"Desire doesn't make us feel good - sometimes it makes us feel downright rotten. That's because dopamine's primary function is to make us pursue happiness, not to make us happy. It doesn't mind putting a little pressure on us - even if that means making us unhappy in the process. ... When your reward center releases dopamine, it also sends a message to the brain's stress center. In this area of the brain, dopamine triggers the release of stress hormones. The result: You feel anxious as you anticipate your object of desire. The need to get what you want starts to feel like a life-or-death emergency, a matter of survival.

Researchers have observed this mixed inner experience of desire and stress in women who crave chocolate. When they see images of chocolate, the women show a startle response - a physiological reflex associated with alarm and arousal, as if spotting a predator in the wild. When asked what they were feeling, the women reported both pleasure and anxiety, along with the feeling of being out of control."