Videogames can evoke the same physiological responses from your mind and your body as the high of many drugs. Most anything is capable of being an addiction for biological creatures such as ourselves - it's simply in the chemistry of our minds that defines what gets us "high".
Many videogames are designed as Skinner boxes of some form or other. Bejewelled was a big addiction for me that I noticed coming and managed to quit before I got too far in - I have [had] a tendency to get "addicted" to games.
The games provide consistent rewards. From the gamers perspective the only down side is leaving the game - you're suddenly tired, hungry, possibly feel unwell. Of course those are the symptoms of sitting on your ass all day playing video games but one isn't aware of the symptoms until one finishes playing (or later, eg trying to work on a couple of hours sleep). It seems all the emotional, adrenaline rewards are in the game and all negative feelings - pain, hunger, tiredness - are outside the game. Such feelings get amplified by having respect within a game for your position, having friends within a game, etc., if one has none of these things in the real world ...
I find it very easy to get in to drink. If I could afford it then it would be a problem for me too I think. Thankfully I've managed to avoid other drugs (except caffeine).
