the price goes up because people buy it and have faith for the price to rise.. the more faith one has in a company the more they are willing to pay for it. The more people buying at asking price the harder it is to get (supply and demand) so people start bidding into faith placing their bid higher than the current price.

Prices go down because people do not have faith in the company to grow and dont buy it and people who are shareholders lose faith and sell... because there isnt anyone buying the stock people cant sell so they reduce their asking price until their order is met by someone who feels that the position does have a future.