The fallacy of composition can be stated a number of ways. The fallacy is—assuming that an action helpful for one person is helpful if all people perform the same act.
An economic example will bring clarity to the issue.
A farmer grows an extra bushel of grain, to increase his income. However, if all farmers grow an extra bushel of grain, all farmers will earn less income.
Why? Because the price garnered by each bushel decreases.
By Pieter Bruegel the Elder – Photo by Szilas in the Metropolitan Museum of Arts, Public Domain