Stock market question, please help?

Okay so i am a business major about to graduate and i still dont understand the stock market completely. i have a problem understanding the reason stocks fall and rise whenever there are good and bad economic news. I mean each stock represents equity from a company, so lets say Company A has 5000 shares worth 10 dollars each. That means the company is worth 50000 dollars. Then, lets say the next day there are some news about the greek bailout going bad and now the shares value falls. This where my question comes in. Why does the share value of a company falls amid bad economic news, if the company itself hasnt experienced any adversity why would their shares fall in value? This question i ve had since i began studying business and after 4 years i still have no answer for it, please someone help out, thanks