A friend of mine sent me a link to a YouTube video from a show that he watches called Mad Men.  It was a clip from an episode called The Wheel and it was quite touching.  Here is the meat of the quote that I found online:

Nostalgia – it’s delicate, but potent. Teddy told me that in Greek, “nostalgia” literally means “the pain from an old wound.” It’s a twinge in your heart far more powerful than memory alone. This device isn’t a spaceship, it’s a time machine. It goes backwards, and forwards… it takes us to a place where we ache to go again. It’s not called the wheel, it’s called the carousel. It let’s us travel the way a child travels – around and around, and back home again, to a place where we know are loved.

Considering our memories as sort of a time machine, is completely apropos.  When we are reflecting on our pasts, we are catapulted into it.  Sometimes it is the way a sunbeam crosses the trees or a song, maybe even a smell, that stimulates memories.  Once they are stirred, we have to revisit those experiences whether they are delightful or painful to recall.