When traders take profits, because of income tax they have less money to put back into the market than they took out.
It follows if thousands and thousands of people have less money to put back in than they took out that share prices have to drop at some stage.
You can thank prohibitively high income taxes on sharetrading for that. Averaging of income not being permitted also has a significant effect in grabbing more money from the average trader and removing money from the stockmarket.
