Why would he unfriend me on Facebook?

I usually don't notice who has friended and unfriended me, but sometimes, when I feel like it, I do go back and check out some statuses. I have this guy friend who has been my fling in the past – we have gone on a week-long trip some years ago, and I have gone and visited him. We live far apart – me on the West Coast and him in the Middle West. We met in a different country while I was working there and him visiting. We had a few brief romantic occasions.

I am getting a little confused now – I was actually gone an internship and that status has shown up on my Facebook. He has messaged me and asked how I was and that I was going to on the internship and said he would like to visit (not me – the country). Then I said he can visit me too, and we talked about future trips, etc. The last time he liked my photo was on New Year's Day. Then I now just noticed we are no longer friends on Facebook. I thought it was strange since he regularly liked my pics (though I don't post much). We rarely talk with each other and we didn't have negative interactions.

I wonder why he suddenly unfriended me on FB? (Note: I believe he has very few friends, possibly only close friends – about 70)