From H.G. Wells, The Time Machine:

It is a law of nature we overlook, that intellectual versatility is the compensation for change, danger, and trouble. … There is no intelligence where there is no change and no need of change. Only those animals partake of intelligence that have to meet a huge variety of needs and dangers.

Where we are too comfortable we need not strive; where we are too complacent we grow fat and incapable of effort.

A little hardship can be a good thing; it separates the important merely from the habitual.