The First Love and/or Love of Your Life
This person teaches you more about love than anyone else in your life -- in the time since, and probably in the time to come as well. They were your first everything, and at the time you imagined they would be your last as well. The world began and ended with their presence, and you never imagined they would cease to be there. But as it does, life started to happen. You grew up, grew apart. One year blended into the next, and eventually you realized you were together out of comfort and normalcy, not love or passion. The ending hurt like hell, but you both knew it was the right thing. If you're lucky, you're still in each other's lives and will always love one another, just in a different way.
The Lesson: Just one? No way. This experience teaches you what love feels like, what passion is, what desire is fueled by. It is your introduction into the world of love.