Upper middle class people can't really do venture capital or private equity (well, they can if they get accredited, but these also require a lot more knowledge and time), bond yields and interest rates are so low, and our retirement system essentially requires chasing inflation-beating yields to work, so they have no choice but to participate in bubbles.
Well known the Fed is pumping liquidity into the market for last few months through the Repo market. Plus cutting the Fed rate twice last year since the recession.
Companies are engaging in stock buybacks. When there’s fewer shares in the market the share price goes up.