Why casual games are addictive: an unordered list.

No instruction necessary: learn as you play; the game won’t eat you alive if you don’t know what you’re doing.
You only need one hand: most actions are often mapped to the mouse; easier to relax while playing; simple controls also make it less intimidating to newcomers.

You’re not gonna die if the phone rings: a lot of the action is dependent upon the player, so if you need to walk away for a minute you’re not jeopardizing hours of gameplay.
If you lose, it’s not the end of the world: little to no chastising for making a mistake; innumerable opportunities to retry a level; no ominous ‘game over’ screens.
The simplest actions become significant: Match two diamonds and fireworks go off; spell a word and it makes a fireball; nothing keeps a player going like the satisfaction of accomplishment; they’re playing for the bells and whistles.
You can play for five minutes: …but you never do; “Just one more round…”

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