Interest rates are negative. This causes people to chase and a big factor in this bubble. Companies are borrowing billions for free to do stock buybacks and keep the price going straight up. Nothing lasts forever. When is the big question.
What does happen, however, is that many modern buyback deals include a bid that gets more aggressive as the stock trades down. So in a selloff, the buyback actually supports the stock more than in a rally. This reduces downside volatility and prevents panic.
The debt problem is a huge issue, but with interest rates as low as they are, it's not entirely crazy to say "we think we will return more in capital gains from buying our own stock than we will be charged in interest for the loan to do so."
https://old.reddit.com/r/wallstreetbets/comments/f4a5kp/how_...
I personally believe we have gotten to the brink of chaos. Foxconn has major issues getting employees back to work, and Apple has shut down stores in China, but AAPL stock is up. AMD is taking market share from Intel, but INTC rallied almost 10% on earnings. Iran launched missiles at a couple of bases housing Americans, and the market rallied the next day. Boeing stock is trading more than double where it was when the 737 Max 8 took its maiden flight. Everyone mentions in passing that equity values are inflated but nobody seems to give a damn.
Maybe people have gotten burned so many times fading the rally during the past three years that they've just given up on trying to call the top.
My money says it's all going to come crashing down eventually. Markets can remain irrational longer than impatient traders can remain solvent, but they have a way of normalizing sooner or later.