Federal Reserve Bank has two mandates, maximize employement and control inflation. They did 3 QEs after 2008 financial crisis to help the employment since the inflation was not an issue. The stock market was a side effect of the QEs since Feds have brought the Fed fund rate down to zero, the savers can not get returns by leaving the money in the banks. What happened was that people started moving money from bond market to stock market and try to get some returns from the stock. Effectively, people are trading low risk for high risk to get some returns.
Now, with Trump's pro-business policies, we have finally seeing some movement in the inflation front. We are expecting Feds will raise the rates several times in 2018. That is why we had a sharp drop of the stock market on Friday.
