After each 4 years the block reward decreases and the miners will get 50 percent less than before and this will decrease the bitcoin available supply provided by miners. Now if you check the supply and demand theory, you will understand after decreasing on the supply the price will rise that's why after each halving cycle, the price of bitcoin can hit a new ath. Plus, there will usually a positive atmosphere and in the bitcoin markets and people will bore more greedy because they saw the last haling and they will take lesson, here increasing the greed will be another reason for the price to rise.