What used to be

We always rush to take photos on graduation, whenever we meet up with our mates just to make sure we'll remember those moments xx years down the road.

Watching the videos from my childhood makes me want to do the same in the future. The only reason why I take photos and videos is for this sole purpose. Actually, if it's something important we wouldn't even forget it (Just like intense dreams). If they're memories with people we aren't close to, do we really need to remember "WE USED TO BE CLOSE"?

Reminiscing the past makes us happy. I feel a sense of loss at times, when I do not dare to ask certain people for pictures. So I'll write about our day out in my diary.

Memories - good/bad. They're definitely things we won't want to forget. We KNOW they put a smile on our faces, make us reflect (just like many other things do). Why, exactly, do we treasure it so much?

Any good, detailed, answer to 'why' with substantiation?