If everyone makes the same bet, it can still lead to disaster (real estate must rise! Let's all speculate on real estate! --> eventual disaster).
Or let's put it this way: a rational bet is something like, this thing is worth $50 now, it will be worth at least $100 later. An irrational bet is one like "this thing will be worth more over and over again!" If everyone things like that, then the thing's price will keep rising until the bubble bursts.
Or let's put it this way: a rational bet is something like, this thing is worth $50 now, it will be worth at least $100 later. An irrational bet is one like "this thing will be worth more over and over again!" If everyone things like that, then the thing's price will keep rising until the bubble bursts.