Right. As someone on the 1st page said, everything is about psychology.
If it's assumed that house prices will keep going up steadily, people will be willing to pay higher prices for houses, because they see the high price as "funny money" (if you're familiar with that concept).
Let's say for example that you buy a $1M house with 20% down payment, and there's an assumption that houses will appreciate 20% year-over-year. That means your $800K mortgage debt wipes itself away after only 4 years.
When buyers are all doing this same napkin math, it creates a bubble.
Houses historically do not appreciate in real terms (fixed costs cancel out appreciation).