I've collected a number of things over the years. Star Wars Lego sets, He-Man figures, coins, and of course comic books. Plenty more beyond that. The main limits on my collections have been money and space. If those were unlimited, I'd keep going until my collections were truly complete. As it is, I have to pick and choose particular subsets to go after. With digital collections though, one of those factors is no longer an issue. There's no worrying about where to put all of your battle pets, or your achievements. Even if you're collecting something that takes up space in your inventory, there's no physical difference in space taken up between one item and one hundred.
Is it any wonder then that achievements, digital trading cards, pets, hats and the like have become so popular in games today? That urge to complete the set or to catch 'em all seems to be universal. It's just a question of what to catch, and how much effort you'll devote to it. Ultimately, it comes down to what your collection means to you personally. So if you find yourself dismissing someone's desire to hunt achievements, or chase pets, or collect outfits for their character stop and think for a moment. You may not be interested in collecting the same things as them, but I'll bet you're collecting something too.