My mom IS a hoarder, and I honestly don't know why she hangs on to stuff. Some things sort of make sense in that they were given to her by family members long gone, but I don't get why it's kept in a closet, unused, for 40 years and now you have no room for things you do use and need. And then there are things that don't need to be kept, but are. (Newspapers, junk mail, bags, etc.)
I loved school and am very social, and I kept all my school notebooks and texts, plus notes and letters exchanged between me and my friends. As an adult I was fortunate to have an apartment with huge closets so I kept these things but they were out of sight. But as I got older and saw my mom's hoard get worse, I started to purge my things. And you know what, I don't miss them at all. I ditched my hundreds of cassette tapes, old texts and notebooks, clothes, whatever. I threw away gifts and clutter I'd received over the years but never really liked. I did keep some scrapbooks from the 80's of bands and movie stars I liked, but I keep thinking I should toss them soon, too. I never look at them except for once a year to show a friend for laughs.
I do understand some people who fear throwing out gifts in case the gift giver asks them where it is. I've never encountered this that I can recall, which is good because I've tossed a lot of gifts. It helps that I've moved a few times so that is also an excuse if it comes up. I regift unopened items or now I put them in the lobby of my apartment building on the "misc table" where other neighbors put out books or items they no longer want, so others can take them.
It used to be hard for me to throw away things like notes, birthday cards, gifts but I think I learned that from my mom because she saved everything. Let's face it, as a kid we saved everything. Shells from the beach, bottle caps, etc. I think that moving around a lot, for home and work, led me to realize it's okay to part with things. The internet also gives me peace of mind because I know everything is available online if I need to replace it again or listen to a song, etc. I do not want to become like my mom. I think that's the biggest incentive of all.