If it costs me more in taxes , why would I file married filing separately ?

Some spouses have had problems with the IRS in the past and still owe the IRS taxes. When you file a joint return, you and your spouse are both equally responsible for the taxes owed on that return. Any refund you might have been entitled to could be used to pay your spouses existing liability with the IRS or the state. Lastly, some people do not want their spouse to be aware of all of their financial involvement.