Its called tax bracketology. If you are married, you have 2 filing options, married filing jointly or married filing separately. When you are married filing jointly and both spouses have income, the combining of incomes pushes you up the tax bracket. Like piling income on income. Think of it like one ladder instead of 2 ladders. If you were single, you both would get the advantage of the lower rungs in the ladder. But if you are climbing that 1 ladder together, only 1 will get the advantage of the lower rungs and the other will have to start at a higher rung when you run out of gas. That's kind of what the tax brackets are like. When you were single, you both got to take advantage of the lower rates as your income climbed up the single rate schedule. But when you are married, there is only one rate schedule so for your combined income, only one of you will get the advantage of the lower rates then other persons income will be taxed toward the higher portion of the bracket.
You can try figuring it married separately and it might help by probably not. Here is a link to the IRS W-4 app that will help you with your withholding for 2021.
