Why do we think stock prices moving up and down?

People expect stock prices to move up and down, and the stock market actually performs in the way we expect. If assuming the market purely react to stochastic shocks, then no one is able to tell what exactly will happen to the market. Under such circumstance, the market price moving up or down is the same as flipping a coin that, and the probability of price increasing or decreasing is also 50%.

Maybe many people have already know that even with three heads in a row, the probability of getting heads or tails does not change is still 50%. This should also be applied to the market. However, due to alternative bias, people tend to shift their choices, though all the outputs are independent of each other. If people have such bias in the financial market (usually many people do), when people are trading every day, they are going to expect the market to fall after the market rises, and vice versa. When there are enough traders in the market making such expectations, the market will act just like what people expect, as there are concentrated bids and asks in the market.

Therefore, when the financial market rises on one day, it adds some downward pressure on the next day's market, and vice versa, due to the existing alternative bias. Of course, the market has many other different types of fluctuations.