Why do people get obsessed so easy?

I've noticed that some people become a bit obsessed if they were dating and got turned down. Or someone isn't interested the person is hell bent on getting them to like them, when they never will. Why don't people just move on instead of getting hurt? Why make excuses for their behaviour as well to lighten the impact of the fact the person doesn't like them?

I have friends like this and it annoys me, like move on an get a life and stop complaining about it and find someone who does like you instead of embarrassing yourself
Why do people get obsessed so easy?
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