What does it mean when a guy says “With you, I always felt like I had to protect my heart from you.”?

This guy and i have been “friends” for 6 months now. We have made out a few times and the attraction is very high and so is the chemistry. We both have came out of serious relationship and we have been each others emotional support for the most part. It’s hard for us to open up about our feelings but i recently started pulling away and he opened up and told me because he knows my situation and how my ex is still in the picture and i still have feelings for him, he always felt like with me he had to guard his heart. This was brought up because he was talking about taking another girl out on a date but not being ready and he also said “even with you like i just am not ready” which i replied with you talk a lot and never shown actions that’s when he said he felt like he always had to guard his heart with me. I didn’t get to ask him what he meant by that because we were interrupted.

What does it mean when a guy says “With you, I always felt like I had to protect my heart from you.”?
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