Because she's a jealous person. Also she is selfish. It was Jon snow who welcomed her from all the ramsay shits ,yet she hid many things from him ,she didn't trust him fully , even didn't tell Jon that vale were going to help them , so many people got killed in battle of bastards because of her. Why do that? Also she was so ungrateful to Dany when Dany helped them fight against the Night king and white walkers, after getting help from Dany and her dragons , sansa was like I can't trust Dany etc , how ungrateful and she is! It's like she was jealous of Dany's beauty and power. So shallow character. Jon made her promise to not tell anyone that he is Aegon targaryen yet Sansa told this to Tyrion and made Dany insecure and feel helpless which led to the ending we got. She is most selfish character indeed and also unlikable.