My first inclination was to say that no one could hope to find the answer to this question through a search engine, because the answer was unique and personal for every one of us. But even as I set out to write that, I realized that it wasn't entirely true; I think that the fundamental answer is the same for each and every one of us.
Here it is: You'd be surprised.
It's pretty easy, in day-to-day life, to take the people around us for granted. It's also easy to give the impression of taking them for granted even when we don't, or to assume that people know the small differences they make in our lives even when we don't mention them--or that they have plenty of other, closer people to affirm them and don't really need to hear from us. It might even be a little intrusive.
But all those people who aren't saying anything for whatever myriad reasons make sense are still there. Friends you haven't talked to in months, or maybe even years, but who know you're out there and value that even if time and life carry them in different directions; the customer who comes into your store three days a week and counts on the extra moment or two you take to chat with her because she's alone most of the time; the person yet unmet whose life you might change entirely.
Someone. That's the answer. It might be someone you overlook, someone you never thought of, someone you've forgotten, someone you don't yet know well. But if you died, it would change someone's life, and not for the better.