Underwear is so important.
If you are wearing a bad set of boxers it can really ruin your day.
If they are too small...ugh. You might as well be wearing heals all day because you will be walking like you are.
If they are too big they slip down even with your pants still on and you have to do this wiggle dance to get them out of where they shouldn't be.
Worse yet, if you get dressed in the dark and put them on backwards...you won't be the same for the next few days after walking around all day with that opening in the back.