Why do some people become very affected by certain experiences and other do not?

I am curious and have been for a while regards this question. I have a friend for example who was hurt by a woman or maybe two women, and since then will never trust another female. Both times the relationships were not very long, nor very fulfilling. In fact they never even lived together. He is like fort Knox. Also he was bullied at school so now really trusts very few people. I have been honored enough to be pulled into his inner circle. Meaning literally “his” inner circle. His thoughts, his dreams his wishes, but it took years. I think the only people he trusts are his parents and me.

I have been hurt many times, abused, and violated but I still trust. I still reach out to make friends. He does not at all. Why did this affect him so much, and me so little? How can I help him? If it all?
(To have a more rounded life).