Why would my ex boyfriend add me on Facebook?

I unfriended him 4 months ago. We run into each other sometimes and say hi and even hung out and caught up once. It's not like he needs to snoop since we've already caught up, and we have a ton of mutual friends if he wants info. And why not add me after that time we hung out if he wanted to be friends. It's weird since I haven’t seen him in a while, and he randomly sent me a friend request yesterday.
Why would my ex boyfriend add me on Facebook?
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