It make me wonder why; like maybe I could have prevented that person from leaving. The person I'm talking about was in my friends list, and I was going to PM him to talk, but I found out he left. I wish I would have contacted him sooner.
That's happened a few times over the years. It's sad to see familiar faces go but there's always more friends to be had until you find those people who you really click with and stay in contact with in spite of you or them leaving the forum.