That's the best question you have asked.
Because the pain of many times being in love confuses them and conflicts them because in this world people do what they want even if they hurt others just to further them selves and this pain teaches us to you caution because not every one is after the true love they seek to sate them selves or their other feelings
everybody know it, experience it but some times failures put doubts to understand,
My guess it's mostly those people who don't believe they've ever experienced love
I think they are afraid of feeling vulnerable.