Why did he reject me when he was single but wants me when he’s in a relationship?

I was seeing this one guy over a year ago. We had a great time and liked each other so when I was asked if he would open to a relationship, was quite surprised when he said he wasn’t ready.
I ended our thing.
Few months later he contacted me saying he’s dating someone but he misses me.
He wanted to see if I open to see him while he’s dating someone else.
What is this all about?

Why did he reject me when he was single but wants me when he’s in a relationship?
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