Sentimental Crap - it's the stuff we love to hoard. The "but it was my son's FAVORITE toy when he was 2.3 years-old!", "This is a picture of my great, great Aunt that I never met before!", "But I got this little statue from umm, you know, umm, that person who came to my wedding." Why do we feel the need to keep this crap?
Second reason: Loving memories. You think possessions that a certain someone used to love needs to be cradled in the arms of of your attic or basement. The "well what if my kid wants that back someday" or "But this was Grandma's favorite pot-holder!" Well chances are, your 19 year-old son can careless about his armless G.I. Joes and your deceased relatives are in better places and know that ratty, old stained potholders are better off in the garbage can.
Third reason: "I will need that someday." I think this is my biggest sentimental hangup with crap. I think of a million reasons why or how I'm going to reuse stuff someday. Like my iPhone box. Normal people throw them away. But I'm all "sneaky" and am like, "Oh, I'm going to save this and use it as the box for my Secret Santa gift next Christmas so the person who gets it will get all excited and think they got an iPhone but in reality, I'm just going to throw pistachios in there and FOOL THEM!" Oh yeah, I'm a tricky one at times.
We need to get over associating feelings and love with possessions. You don't need to save your Grandmother's pot holder - you can toss that thing and have your memories of her with her potholder. Maybe you can go through old photos and scan ones of your Granny with her potholder and throw them on Facebook or another, more private photo site. Then when you get the urge to think or remember her, you can look at photos instead of the potholder.
Don't feel guilty about throwing away gifts - Do you REALLY think my mother still has the Clapper I was so proud to give her 20-something years ago? No. She probably doesn't (but one never knows). But the more important question: Do I really care if she has it? No.
So I started de-sensitizing myself from my crap. One big accomplishment I made: The Unity Candle.
We had some candles in my wedding back in 2001. I saved some for who knows what reason. So when I unwrapped this big, white pillar candle, I was like "Why the heck am I saving this?" Was I going to serenade my husband with it lit on our 50th Anniversary? No. Would my kids want it someday to use at their wedding? No. So why has it been hanging out in a box for 12 years? Beats me. That sucker got burned.