Why do we save?

With all of my packing, I also had to un-pack the attic. When Ryan and I moved into this house, we had been married for less than a year. I still had stuff at my mom's place. A lot of so called keepsakes ended up in this attic. We were 2 people combining keepsakes. So now,we are trying to pack things and move, some stuff has been in our attic from the beginning. It had been up there for 6 years. (Or so). I came across some of my stuff that for some odd reason, i saved. Now, I have no idea why I just didn't get rid of them at the time. Since I haven't seen most of that stuff in 6 years, I am more than ok with getting rid of it. Well, most of it. There are 2 very full boxes of stuff that Ryan needs to go through. Some of it, I can see him saving. Most of it though, I wish he'd just toss it because once again, it will go into the attic never to be seen again for more years than I can count.
I wonder why we save some of these things. Why do we see some things as keepsakes and other things as trash? My idea of a keepsake is often different from Ryan's idea of one as well. Photos, yearbooks. A few things from the kids. That's about all I am interested in saving right now. I'm sure his list is much much longer. I have found myself getting into a habit of getting rid of things before they have time to sit around. It drives Ryan insane. The clutter of things is beginning to get on my nerves with each year that gos by. I'm going to be an old lady, living by myself, with bare white walls and nothing to sit on, haha! Like as soon as the kids outgrow something, I'm ready to sell it or donate it. Now! Before it has time to sit around and get shoved into a closet somewhere. If I could manage to be more organized with the crap that we do have, I think I would be more relaxed about having stuff stick around for a while longer. But I have a problem finding a place for things. Finding some where for each thing to be, and be out of my daily way. Since I'm horrible at organizing, my solution seems to be to get rid of it. Right or wrong, it just seems easier I guess.
How do you decide what you should save and what you should get rid of?