Almost all falls in the market are due to just two things: either the market is thought to have become riskier; and/or growth expectations fall.
Let's take the first case first. Share prices fall when risk increases simply because investors need a higher risk premium to compensate them for the greater risk - and this can only be obtained if shares fall to a level from which subsequent returns will be higher. For the average investor, this is a wash: the unpleasantness of higher risk is offset by higher expected returns. He should not, therefore, change his asset allocation. The same applies, in reverse, if shares rise because of a reduction in perceived risk.