When we lead quiet lives, we might get the impression at times that we aren't making much of an impact on other people. However, one underappreciated reality of the world is that we have a way of influencing people without even realizing it.
We may have thought we were just passing the time when we had a chat with someone, but when we hear that person quote us later on, we realize the effect our words had. In such situations, it can be surprising enough to hear that somebody even remembered what we said, let alone took it to heart.
So if one vet ever had any doubt about the mark he left on the world, it likely disappeared when he saw how many people were excited to mark his 100th birthday.