No matter how we live our lives, we're ultimately limited in how much we will experience by our natural life spans.

I see every interaction with someone else as an opportunity to peer into another life. A life I didn't live. A path I didn't take. Even if that path feels to you completely pedestrian, there is still a whole life there that you didn't live.

Even if they haven't examined or reflected on their life, you can (with respect, and perhaps best silently to yourself :-) ).