People who lost someone, people who are separated from their loved ones, people who are introvert but must act sociable, people who are depressed but must act chipper or else be asked questions, people who are alone but see all other people getting together for parties, people that hate their families but cannot escape the obligatory gathering, people who are poor but feel pressure to give gifts and have a lavish meal.
If you're depressed or something very bad's happened in your life, the contrast of it being a time of celebration and seeing everyone else happy can be especially painful. Kind of like being forced to go to a jubilant party where everyone's singing and dancing the day after your best friend died, for example.
edit: This is a legitimate question, in case it wasn't obvious.
edit 2: Got it, no questions allowed. Apologies.