why was blair so upset?

When Chuck slept with Jenny, why was Blair mad at Chuck?

Her and Chuck had been broken up. For quite a while. So why was she mad? Yeah, he had done shitty things to her, and her him. But still, that particular plot made no sense. Why would you be mad at an ex for sleeping with someone else for comfort?

Also, Jenny had a right to be upset and regretful, but no one had to right to be mad at Chuck as he asked multiple times and they were both sober. I die on this hill.