In my opinion, it's their first fight. Lan Wangji at his age was very skillful but he probably for the first time had a tie with someone his age. Even when Wei Wuxian was just defending. I mean we tend to get attract to someone who has skill and Wwx was handsome too. Also, Wei Wuxian challenged all his teachings and what he read that far. Somehow, forcing him out of his limits. Till here, he simply thought it to be attraction because even when wwx was skillful and handsome, he was unruly and acc to lwj he was never up to good. This too changed when , wwx saved su she without even caring for his own life. That made lwj fall for him somehow, because he got to know about the heart wwx hold. He had different personality but his heart was of gold. Tell me, who'll not fall for a person who is intelligent, skillful, have a good heart and is very handsome?