What is the psychology behind addictive video games? Is the psychology the same as in casino games? where the person is addicted because winning is so unpredictable?

Are RPG games addictive by Gestalt theory, where players mind likes to see things organized and being whole and completed as opposed to being fractioned and incomplete?
People play games for different reasons - check out the Bartle test which describes some motivations behind actions in games, particularly multiplayer RPGs.

I've written an ebook on video game addiction, and will attempt to illustrate factors that contribute to a game being "addictive".

There are a few types of most addictive video games 


Massively Multiplayer Role Playing Games (MMORPG like World of Warcraft)
Competitive shooters. (like Call of Duty or World of Tanks) 
"Build and steal" strategy games (like Clash of Clans)
Purely social/achievement driven games (like FarmVille)


For example World of Warcraft, has been described as a "theme park MMORPG" - playera can do a variety of activities and if they get bored, they can do something else. The social component is a big part of a person wanting to return to this kind of games.

Then there are shooter games that are played in short matches, for example World of Tanks, War Thunder. Games like these depend on persistent RPG character progression to artificially create inequality among players. For example in a 32 player tank you can be driving the weakest tank, pretty much being prey for everyone else. This creates a feeling of "this is unfair!"

Part of the addictive nature of these games is that player is forced to "grind" - play the game over and over again, frequently without any enjoyment. This is a way to get better tanks/planes/guns/etc.

The way "grinding" is done in competitive games is by playing short matches (3-7 minutes), typically very adrenaline driven engagements. Once an engagement is over, a player is left in a state of imbalance - the player wants to experience the rush of victory again or is dissatisfied with defeat and wants to win instead. It's difficult to get off this cycle, which is why such games are called addictive. A player can spend entire day doing these short matches while looking forward to a more powerful tank/plane/gun/etc.

In some ways, this random element with inequality and short matches can be compared to a casino slot machine on steroids. Instead of pulling the handle, a player presses the "Battle" button, which is results in the player placed in a random battle. 

Finally there are a number of simple "build and steal" kind of games, like Clash of Clans (smart phone games). A player builds a settlement, but resources are very scarce. A player is encouraged to steal resources by raiding neighboring settlements.  These games also thrive on the feeling of inequality among players The inequality gap between players can be lessened by paying real money - "Pay to Win (P2W)" model.** 

"Pay to Win" is very much like gambling, and companies like Apple and Clash of Clans developers are taking heat for allowing 12 year olds spend real money in games to cover the inequality. It has been calculated that to buy everything in a game like Clash of Clans, a person would have to spend 11000 $$USD or close to a year of real time waiting for the resources to build up (all the while being raided by stronger players). Top of the line planes or tanks can cost up to 200$.

Once real money are invested, different mechanics come to play - it's hard to abandon a game where you invested hundreds of hours or dollars.

Most of this post is based on my own observations and analysis, as there isn't much mainstream interest in understanding the addictive nature of video games.