Why Do I Feel Bad When Someone I'm Flirting With Is Also Flirting With Other Guys?

As the title: I (26M) feel bad when a girl (25F) I've been actively flirting with talks to other men. I know that's completely normal behavior, but I can't shake this feeling. Why do I feel this way, and what can I do to understand it better? I think I need to manage this, because in today's world, it's very normal.