i keep everything for a while, then weed it out and pitch that which has lost it's importance! but...a couple of things never leave!
my great aunt gave me her engagement ring when i was about 19, after years of telling me that i would inherit in when she passed away (she didn't have any children). that ring became a source of many arguments, since my grandparents (great aunts brother and sis in law) had used the ring at their wedding ceremony when they couldn't afford their own. they tried several tactics to get it from me, even after great aunt died, but i held firm. long story short, i gained weight, lost weight, the band broke and dh had the diamond remounted in my engagement ring...62 years after the ring was originally bought!
the other thing i have held tight to is a 8" tall stuffed ET. i've had it since the night, 17 years ago, that one of my best friends (and sometimes more, lol) was killed on his motorcycle. i was working 2 jobs at the time and had to get to the second one while they were still working to save him at the hospital. another guy that i worked with at the first job came to the bar i tended at for a second job and stayed until we closed. he won the ET for me in a claw game and i always connect it with the day that i lost a best friend and someone else showed how true their friendship was. i still sleep with ET under my pillow!
so, yeah, we're all sentimental in some way; often the most surprising things become keepsakes!