William, his friends and you have went to see the a film together, William didn't knew much about the film so he couldn't really talk about it with you and his friends.One of his friends, Ranboo do knew things about it and You were talking and having fun with him. William was a bit jealous of that since u didn't pay attention to him so when u guys went home together(Because you lived together),He was acting grumpy and not paying any attention to you