If you have ever tried to throw away old toys or old school work in front of your kids, chances are, they whined and cried and told you, "Don't do it!" Why? I mean, at the ripe old age of 4, have they already been conditioned to want to keep all the crap that surrounds their life?
I'm also noticing with my older one now - he just wants to buy stuff all the time. Like Legos or these guy things - he's always like "when are we getting a new set!" or "I want a new one!" Blah. I always respond with "Go play with the crap you already have and be grateful for that." But it's met with ill reception.
I'm no psychologist but it's probably due to the caveman instinct in all of us to keep crap. Heaven knows, the kids have seen me throw away a ton of stuff - so it's not like they are seeing me wanting to keep tons of crap. I'm working well with the older one - he understands that he doesn't need all his baby toys anymore. The younger one is still very "no no no! We need to keep it!" They both have so much fun in the attic when we go through old crap. They revive some stuff or find "cool" things up there. But it's still pretty much the land of unwanted toys and crap.
All I can really do is keep instilling in them that material possessions aren't that important. Even if they don't believe me or don't listen. I want them to have parents that have an attitude that "material things don't matter". Make them realize that money has value in it and what you do with it is up to you. Are you going to waste it on stuff that you use once and throw it in the closet or are you going to use it to buy things & experiences that matter? I've also heard my 6 year-old say "I don't want to buy that at the Dollar Store because it's just going to break." Teaching them quality over quantity is a great lesson too.
So hopefully all these little tidbits won't make them psycho and become Hoarders later in life. At least they'd have a chance at mini stardom if I have to call A&E to have a hoarding intervention on them.