We put down my childhood dog in early high school. I remember reading your original post way back when and really feeling it. My dad and I still talk about her - he's careful to not talk for too long. His eyes get watery; mine too. She was there for everything.
He's had 5 dogs since, 3 still living. None for me, life circumstances and my own fear haven't arranged for it.
My wife and I are thinking about getting a dog soon. She never had one growing up - she doesn't know what she's about to get herself in to.
I don't know what else to say except I know that pain.