"oh, please. you can’t REALLY be in a great mood. there are people out there who just got raises. people just got married. people are being reunited with their families right now. how dare you say you’re happy."
The equivalent of someone saying you can’t be sad because other people have it worse than you.
Candles: person A has a kink for candles. He/she also believes that anything can be a dildo if you are brave enough. They try out this theory on person B. They end up in a hospital.
Candy canes #1: character A posts a picture of candy canes on his/her account on a social platform of your choice. 5 minutes later, he/she receives anon hate messages from internet social justice fighters ('Candy canes? Are you disrespecting disabled people?!?!!? I bet these aren't vegan candy canes!!!!!'). Person B tries to defend person A, but both of their accounts get blocked and can’t send Christmas messages to 400+ of their friends.
Candy canes #2: Person A uses the line “Is that a twelve-inch candy cane in your pocket or you’re just happy to see me?” Person B dares person A to find out. Turns out it is really just a massive candy cane.
Candy canes #3: Person A gets arrested for handing out free candy canes to kids and Person B has to bail him/her out. But Person B doesn’t bail person A out and Person A spends Christmas in jail. They never see each other again.
Christmas tree #1: Your characters buy a tree for their flat, but unfortunately it’s infested with spiders. Both of your characters are afraid of insects and none of them is willing to either get rid of the spiders or the tree. They spend Christmas in a hotel room.
Christmas tree #2: Person A ties person b up in a Christmas tree and forgets all about him/her because person a becomes busy preparing Christmas dinner. Person B is not happy about it. It’s their last Christmas together.
Christmas ornaments: Person A tells person B that he/she will spend the evening on making Christmas ornaments. Person B comes home earlier that day, only to find person A playing with person B’s best friend’s balls.
Fireworks: Character A decides to prepare a small firework show for character B. Unfortunately, they get is a ticket for disrupting curfew and scaring animals living in the park.
Snow: Your characters’ car get caught in a snow storm. It’s a not a cute fic wherein your characters cuddle/share body heat/shag and survive the cold night. Let’s be real here. It’s too cold and they die.
Wet socks: person A has wet socks and forgets to change them, which results in a cold. It’s not cute or adorable and there’s nothing nice about it. Instead of leading to person B fussing over person A, it leads to general annoyance and misery on both parts.
Wrapping paper: person B’s cat destroys all the wrapping paper in the house. Because fuck you, that’s why.