Why he thinks no one will ever be interested in him again?

I have older brother in early 20s. He was never the type who would chase girls and always okay being single. He had one relationship like 5 years ago. Like 10 months ago he was seeing one girl he really liked. She liked him too and was crushing on him for a long time. It looked like they will be official any time soon but she ended things between them. It really hurt him. I told him that he will find someone he deserves but he always says something like who would be interested in me, if she rejected me the rest will too and stuff. I understand it is hard one to swallow but there were many girls who were interested in him especially when he was younger. But he looks like he gave up.

Why he thinks no one will ever be interested in him again?
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