There are numerous articles and proof that getting rewarded in games causes positive chemical reactions in the brain. Here are some interesting articles on the subject:
{|* | [File:Skinnerbox.gif|400px]| [File:Theory-of-Gaming-Motivation.png|400px]|-| [http://www.mostdangerousgamedesign.com/2013/08/the-psychology-of-rewards-in-games.html]* | [http://www.thinkfeelplay.com/theory-of-gaming-motivation/]|}