Anchoring bias, a cognitive bias where an individual relies too heavily on an initial piece of information (the "anchor") when making decisions, often comes into play during bull markets. Investors who have experienced bear markets might anchor their expectations to low prices and see current prices as 'too high,' even if they're rational within the context of the bull market. This results in them missing out on potential gains.

Further, investors are often swayed by loss aversion, the principle that losses have a significantly greater emotional impact than an equivalent amount of gains. So, while a bull market offers the opportunity for significant gains, the potential losses that could result from a market downturn are a substantial psychological deterrent.