Why do we get a tax refund for "married filing separately" but have to pay quite a bit when we file joint?

Filing jointly can cause a difference in the tax calculation as opposed to filing separately. This is often referred to as 'the marriage penalty.' The most important itme to be considered is the use of the itemized deduction. If one person itemizes, the other must also itemize. If the second person cannot itemize, they are not allowed the standard deduction either. Dependency exemption of children can be allocated between the spouses, but not duplicated on each other's returns.