The key points are:
Cyclical fluctuations are a key aspect of investment markets. Most are driven by economic developments but get magnified by swings in investor sentiment.
Of particular importance are the long-term cycles which are often driven by waves of innovation and the 3-5 year business cycle. Lately we have been in the benign phase of the business cycle and may have be entering a weaker and constrained phase of the long-term cycle.
Periods of poor returns invariably give way to great returns & vice versa. The key is to not get thrown by them.