Also share prices are a bit like house prices. Both are only worth what someone will pay for them.
Both the stock market and housing market rely on people having confidence in them.
Sometimes there is "confidence" in the stock market, and people will buy shares (which drives prices up), sometimes people LOSE confidence in the stock market and sell shares (which drives prices down).
This confidence can come and go for all sorts of reasons, such as interest rates, world economy, even the millenium had an effect (many people sold shares before we went from 1999 to 2000 because they thought the millenium bug would cause a stock market slump).
Of course the great art is buying shares when they are low and selling when they are high. Crack that and you have it made.