It's hard to describe the feeling this produces. Getting a sense of closure is difficult. You may wonder how the person died and if you could have done something to prevent it. Maybe it was a suicide or drug or alcohol related. What if, years ago, you had said something like, "Remember to get help if you need it," or been able to provide some valuable piece of advice. Maybe if you had just stayed in touch.
We should create a day to commemorate these things. We could call it Old Departed Friends Day or something like that. We could honor the memory of the departed and forgive ourselves for allowing a cherished friend to drift out of our lives. We could resolve to be better, to reach out to people we once knew. Of course, there may already be such a day--there seems to be one for everything else. I'll have to Google it.