It is illegal to lie to the government.
John is filing his taxes and is filling out a form.
John wants to respond to the line item asking for his total income.
John states his actual working income as shown on his W2.
John understates his income significantly, to avoid paying more taxes.
John feels good about helping the country grow and pay for all services.
The government audits John, causing him to have to pay a lot more in the end due to fines and penalties.