To prove that you're jealous and still have feelings for your ex? Don't waste your time with him. Some best friend that goes behind your back to be with your ex that you were in a serious long term relationship with. That's no friend. Just rid him from your life. Cause he might say something about his relationship with your ex that is inappropriate and will get on your nerves.
I try not to get jealous in relationships. It's something that I'm actively working on. Having being cheated on in the past, I know that it's a piece of baggage I bring to new relationships that is important for me to leave behind. However, when my significant other is friends with their ex, it makes me nervous. For many, it is not.
You might say no, but science says yes. Here's how to break your addiction and start feeling better right now. Even worse is watching your best friend start trying to put moves on your ex. To answer the titular question, you should be pissed if your ex-girlfriend shacks up with your best friend, no matter what his or her excuse is. Two of the closest people in your life have gone behind your back, and your best friend has crossed a line.