What Defines You?

I took a Sociology class in college that asked us to write down all the titles we define ourselves as, for instance, daughter, friend, student, etc. As I experience life, I've noticed I define myself also by events such as where I live or an event that has happened to me. And as time moves on I notice that we, most people, define our lives by tragedy.
My family experienced a fire. And I have found that when meeting someone new it seems I bring this event up at some point early in our meeting. Why? I didn't lose a love one, my house has since been rebuilt. I don't want to be defined by this event but I can't seem to help it.
I say this because I was struck by a moment the other day that has left me puzzled. A mom, that I recently met and have spoken to a few times over this past school year has chosen to not define herself by tragedy. In the various conversations we've had they've revolved around children/family. Our sons happen to be in kindergarten together. She has other children, younger. So, of course conversations lead to talk about her other children, ages, gender, etc.
The event that happened was coming across a newspaper article posted at our pediatricians' office which had a photo of this family and a charity event the father was participating in because of the death of their first child.
WOW! Never once in the conversations we had had did the topic of a first child or losing a first child taken place. What an amazing woman! Here something so tragic, so profoundly painful, something that would hit you every day you woke up and choosing not to let it define you. I was blown away.
Why as a society do we make anniversaries for tragedy? Do we want tragedy to define our life? When do we make the decision to define our live by happiness? Celebrate life not death?