WHY DO SOME OWE MONEY AND OTHERS GET A REFUND FOR THEIR TAXES?

i enter my taxes every assemblage no concern how lowercase i attain but i ease dont see how doing your income taxes work. how become whatever of us owe money and others intend a refund? if there’s an turn in the federal withholding doesnt that stingy you should automatically intend whatever money back? my swain has worked part-time at the aforementioned employ for more than 3 yrs and he whatever how has unpaid money to the bureau every year. how become he gets null back? in what conditions module digit owe money and in what conditions module digit intend a refund?