It's true what people say: If you want the most reward, you have to take the most risk. But prudent investing is about taking calculated risks, not blind ones. And after three consecutive years of hardy stock returns, it may be time to dial down the risk in your portfolio, in preparation for the eventuality of a market stumble.
Or your circumstances in life might dictate a more cautious stance, for whatever reason. De-risking is about planning ahead.