I remember in my college philosophy class reading about this guy who believed that all of reality is constantly changing. Because of this, according to this fellow, it is impossible to step into the same river twice.
Another guy came along and said yes, reality is constantly changing and, because of this, it is impossible to step into the same river ONCE.
The implications of these lines of reasoning are legion. But right now, being sufficiently inebriated enough to contemplate different thoughts, yet not enough to be stupid, I believe it tells us that while everything is always changing, the essence of things remains the same, so each individual smile is eternal.
Which is why dinner with friends is absolutely pure gold.