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.
I like the clarity an economic example brings.
A farmer grows an extra bushel of grain, his income increases by the value of one bushel.
However, if all farmers grow an extra bushel of grain, all farmers can earn less income.
Why? Because the value of all the other bushels can go down further than the value the extra bushel of grain earns.