Addiction
I cracked the code!
By playing games, I found it out. You are never done! Someone is better, something is better, and also you can restart, just so you can feel that rush again, it is like a drug. This is what I think makes up successful games.

Items
My first point of addiction is items. In TF2 (Team Fortress 2) there are so many items. Weapons, cosmetics, taunts, then the subcategories of unusual, strange, kill streak, australlium, and festive, and then more sub categories for unusual effects, kill streak types. This creates an addiction like a drug, and to make it a more potent drug they have trading, and this creates an economy, where you can literally make money off of playing a game for fun, the currency is buds, keys and real money. With this you play so much so you can get new items, trade those items for tickets, keys, buds, and more. With a ticket you can play a new gamemode and get, australliums, killstreaks, unusuals, and of course stranges. With these, even if you are bad at the game, people will bow down to you and your awesome items, you gain respect, and when you are bored of the game, you sell you backpack for hundreds to thousands of dollars, from just playing a game.

This was bought for 540 buds, that's $13,392
Gotta buy em' all
In Starbound, there are random weapons, tiers, tech, and ship upgrades. There is DPS (Damage per second), energy per shot, damage per swing/shot. Then weapons are random, and get better by each planet, you start off on a gentle star, you find a weapon with 50 damage and you think you just found the best weapon, then you find an 80 damage weapon, then on the last planets you can find a rocket launcher with a thousand damage per second and 1,500 per shot, then you think you have an amazing weapon, but you know that there is always more, and you are never done. With armor you can gain more health, armor, or energy. With tech you can gain more mobility. With a bigger ship, you can store more, decorate, add living rooms for an npc (non-player character), pet rooms, and more.
So much room, "okay, my imaginary friend's bed will be there..."
Exploration
In Starbound you five different stars, gentle, eccentric, radioactive, frost, and fiery. Each has its own type of solar system, and to get to these solar systems you must do quests. In quests they require you to explore. The first quests requires you to be able to get to the core of your planet, and collect core fragments. This takes time, lots and your mind is so centered on it that you loose track of time, and while mining you run into caves, and you feel like it is you duty to explore them, inside are chests, monsters and traps. You must fight stay focused on your surroundings, and when you find that weapon or ore, you become overjoyed, this makes you want more, like a drug, and this is its climax and it lasts long enough for you to do the same thing and it keeps happening, you never get bored. You then find the twenty core fragments, your next mission is finding a way up, but you are still on this rush, so you do go up but you look for caves you want a cave, you want that rush. five hours just pass without you even realizing it. Wen you beam up to your ship and repair its fuel tank, you find out your not done, its FTL (Faster than light) drive is broken, you can still travel within your solar system, but now you need to go to the gate. The gate brings you to an outpost. In the outpost you find many missions, you must complete all of these so you can complete the mission that provides you with the materials needed to repair it. So you go to the place it sends you, you know your mission, but while you are doing subconsciously, you want all the items, you want to kill all the enemies to get more pixels, you find a secret room you think your a genius, you find the boss room you think your a genius. You defeat the boss, you think your the best. This is a cycle, and it never ends, even when your done, you have not explored all the stars. With randomized enemies, weapons, and ores, you are never done, and by some chance you get bored, you can create a new character and do it all again.
SO MUCH TO EXPLORE!!
Multiplayer
This isn't necessary but it is good to have
Okay, I will admit I haven't played this for a while but this was my analysis of portal's co-op. AMAZING! Team work, so much team work. It drives you to insanity, but that insanity gives you and your partner the drive to finish. You are yelling at your partner, but when you finish that stage you press your taunt button and click hug because you are so happy, and all of the sudden everything is resolved, your both happy like the whole not going into your portal thing never happened. It creates such joy, and anguish. Also in co-op you get laughs out of it, being able to talk without anyone around, and not be judged, its awesome.
Nyahh... five seconds ago they were saying that their partner is stupid