The urge to collect all the items, achievements or secrets in a game.
"World map has completion bars for each area that offer rewards. Events and limited time cosmetics for the player character / heroes."
"pieces for albums you earn from card packs you earn from loot bags (see my review on loot boxes)"
"during events, they have many sets to complete. Some are just standalone buildings that can be upgraded and others are things you need to assemble together to create a set of something."
"Getting all of the free songs in the base game unlocks Unlimited Play until they add more content. This is essentially a 100% collection award, providing something to work towards. However, they made this goal nearly unobtainable for casual players with the Deluxe update. (Deluxe songs take more than double, even quadruple the time of a normal song)"
"heirlooms, challenges, accessories, artifacts, careers, achievements, treasures, designer clothes, ribbons, scavenger hunts"
"Stickers, which you can also buy with money. Has special golden sticker that are required to complete the collection but you can't trade them with friends, only randomly earn or buy loot boxes that may contain them."