Even older and studied investment a bit. I'd say most people don't think "stonks can only go up" but do think the markets are a random walk with an expected return of 7% or so pretty much regardless of when you buy. But that's not true - markets swing between expensive (say 25 PE) and cheap (say 7 PE) and they mean revert. As a result if you buy expensive your expected 10 year return is not much (say 0%) and if you buy cheap it's a lot (say 12%). Jeremy Grantham / GMO are good on that stuff.