Idk how I should feel...

So this guy and I have been talking/vibing for a few months now, dating to date but we were hanging out the other day and he tells me that he isn’t sure if he’s mentally ready to be in a relationship. I get upset and I ask him why and he stated it’s fear because of how he got his heartbroken by his ex and then he was going to tell me what happened (bc I’m nosey) but I told him he didn’t have to if he wasn’t feeling up to it. We both have clear/open communication about how we’re feeling about how we are going to be as a couple, but now I’m feeling insecure that bad news will always be around the corner. He reassured me that all is well and I reassured him that he doesn’t need to fear what he could have w/ me. But does anyone understand why this would make you feel insecure in moments?