I designed a game in my spare time where users collect cards representing units, form decks with those cards, bring the decks into a battle, and buy more cards with proceeds. While designing it, I started thinking about how I can get the user to form an emotional attachment to some of the cards.
Games like Magic: the Gathering already have a few features that causes users to form emotional attachments- a very rare card has a greater value than the rest, therefore the user has more emotional investment in this card than the others in her deck. This is a fairly weak emotional attachment, but with a bit of work, you can create strong emotional attachments to in-game items.
For the game I described above, I wanted to tie the emotional attachment of a unit to the users interactions with it. Interactions such as the amount battles the card has been in, the amount of kills, deaths, raw damage, etc. I wanted to use these stats because emotional attachments are naturally formed by a persons experiences, so building mechanics that leverage this are naturally compelling.
How do we do it? As the units usage stats increases, you reward the unit. Not the user, the unit.
In my game, I rewarded the unit by giving it a unique identifier- in this case a better name. For example, a deck has five "Soldier" cards, and during gameplay one gets seventeen kills and is promoted to a "Hardened Solider".
The progression from a common Solider card, to a unique Hardened card, is where the user forms the emotional attachment to the unit. The name anchors the memory of this play session to this specific unit. And now every time the user sees this special card, they remember how much fun it was in that game they played. Positive emotional attachment achieved.
But we're not done.
If the user has a positive emotional attachment a particular unit, they are more likely to use it again. As they do, you start buffing the unit so that its' real value- its' attack, defense, etc. starts to reflect the users perceived value of the unit. If the unit crosses a certain threshold, immortalize the unit- make it a Hero, a Legend, a lasting positive experience with the game.
But you don't make immortal immortal, as in can't die. See, all things must die, unless you monetize.
If this unit dies, you present them with a one-time only screen of "revive for $2.00 worth of premium currency or else your favourite thing in the game dies". Are you evil for doing this? Yes. Do you lose users at this point? Maybe, but if you buy $10.00 worth of premium currency right now you get an awesome rare care guaranteed.
This process of creating emotional attachment to in-game items puts to rest two questions: "How do game developers make money off their games?", and "Can games make you cry?" Not only does it make your game more engaging, but it makes it more engaging over time. Forming real emotional attachment to a virtual item is not easy, but can be critical to having a successful game.
Good luck out there.