After three good years, 12% a year looks reasonable. After one bad year, equities look like a structural mistake. The underlying data changed in neither case: what changed is what is freshest in memory.
It is what drives money into funds right after the best runs and out right after the worst, and it explains much of the gap between a fund's return and the return its investors actually receive.
The correction is boring and it works: look at long series rather than the last twelve months, and write down your return assumptions once rather than revising them because of what the market did last quarter.