I'm always curious why some apps are addictive.

If you want to read a loooong article about why Angry Birds is, see: http://www.mauronewmedia.com/blog/2011/02/why-angry-birds-is-so-successful-a-cognitive-teardown-of-the-user-experience/

Here's my summary:

• Simple but engaging basic interaction.

• Response time is cleverly managed. Performance matters.

• An element of mystery.

• Short term memory management.

• A distinctive visual appeal.

If these were easy to put into a single application, there would be more addictive applications.

Unfortunately, it's not easy, so don't regret trying. Aiming for those characteristics is a privilege.

One of the best analysis on how Angry Birds (of any other piece of software ever written and analyzed in details) does short term memory management. I learnt so much.

Trigger-action-reward

That's about as simple as you can make it. It's the basic formula for habits (good and bad.) 

Trigger: what makes me want to initiate?

Action: what is the thing that needs to be done?

Reward: what makes the person feel good after they do the action? 

Throw in a leveling up system and you have yourself a sticky game. 

Add a level of randomness and you have a billion dollar casino empire.

Well said.

Can you imagine how addictive Angry Birds would be if they added a gambling element to it?

Hysterical. We're just talking about this and BOOM: article on techcrunch about apps and habits/addiction...

http://techcrunch.com/2012/04/22/the-billion-dollar-mind-trick/