Come on - admit it. Let the cat out of the bag.
In your private banks and bags, you may a few sentimental items. These are the objects or letters you just can't delete; for whatever personal reason, these items hold emotional value.
Mat has a few such items. She blushed when she admitted this to a player friend a few weeks ago; however, he confessed the same. The difference being his objects and hers were weapons, commendations, and the like; hers were some flowers, a hand-crafted item or two, and a few of the funniest or sweetest letters. (Now you might suppose that Mat's human is equally as sentimental, and on some level she is; though she has been known to bulldoze through a garage or closet, and remove clutter and junk without mercy or remorse, she has held onto a few treasures along the way, too.)
If you are the sort of player who only keeps the most pragmatic of items, you have Mat's admiration. This is a quality she will most likely never possess, the ability to streamline and focus solely on making gold and gear collecting (and again, admittedly, both worthwhile goals).
This may be a myth, but they say that a goldfish will grow to fit its pond size; and just like the items we collect, the bigger the bag, the more wiggle room we have for the emotional or entertaining items. One comment that stood out for Mat at this past year's Blizzcon was when a (female) player politely asked if, because she really enjoyed the holiday achievements, if Blizz would consider adding a holiday closet tab to the bank vaults, so she could store her items in an organized way, without taking precious space from the gold-making ones. The response was an emphatic: "This isn't World of Dresscraft!" Ouch. It isn't? If Mat took a random poll right now, right here, of how many players would like her to trash her Santa suit or pirate outfit, she imagines quite a different sentiment.