The culprit is dopamine — Dopamine was "discovered" in 1958 by Arvid Carlsson and Nils-Ake Hillarp at the National Heart Institute of Sweden. Dopamine is created in various parts of the brain and is critical in all sorts of brain functions, including thinking, moving, sleeping, mood, attention, motivation, seeking and reward.
Pleasure vs. seeking — You may have heard that dopamine controls the "pleasure" systems of the brain: that dopamine makes you feel enjoyment, pleasure, and therefore motivates you to seek out certain behaviors, such as food, sex, and drugs. Recent research is changing this view. Instead of dopamine causing you to experience pleasure, the latest research shows that dopamine causes seeking behavior. Dopamine causes you to want, desire, seek out, and search. It increases your general level of arousal and your goal-directed behavior. From an evolutionary stand-point this is critical. The dopamine seeking system keeps you motivated to move through your world, learn, and survive. It's not just about physical needs such as food, or sex, but also about abstract concepts. Dopamine makes you curious about ideas and fuels your searching for information. Research shows that it is the opioid system (separate from dopamine) that makes us feel pleasure.