The short version:
The stock market is nothing but a measure how how much richer the 0.1% think that they’ll be in the future. Because rich people can’t conceive of a world in which they can rightly lose their money, the stock market will tend to go up until even the most thick-skulled trust fund baby realizes that things are about to go bad.
ETA:
This is of course the market as a whole. There are plenty of reasons to make a single stock go down: it makes a hostile takeover easier for example.
