Have you ever thought about this question? Why do you play D&D? The most common answer would be "because I enjoy it". But lets stop and think for a minute (as psychologists do!) and imagine what Freud could say. Using psychosexual analysis he could suggest that we displace our hidden desires and wishes onto our characters. This could especially be true if you play your character as trying to hit/sleep with someone, as this is your primative instinct to mate and reproduce. He could also suggest that it's some form of escape, to get away from reality and enter a fantasy world because the mind becomes damaged.
Another area of psychology...behaviourism. Behaviourism suggests we have no mind, but we learn associations by the use of classical and operant conditioning. To put it simply, if we do something and get rewarded for doing it, we are more likely to do it again. For example, if you paint a picture for fun, and then get paid for it, you're more likely to paint another picture again in anticipation of getting rewarded. Applying this to D&D, behaviourists could suggest we find it rewarding. Whenever we perform actions in the game such as killing a monster, performing a quest or helping someone out we get rewarded for it, usually by experience points or money. We can use experience and money to develop our character, which becomes more powerful. These two areas (gaining money & experience and developing your character) is rewarding, and therefore you are more likely to do it again a.k.a. play the game again.
Humanistic psychologists may say that we play D&D so we can develop who we are. As we play different characters we can explore the different personalities that different people have. If we like certain aspects of the characters, we will apply it to the self (because the personality characteristic is believed to be benificial e.g. to be able to lie or bluff convincingly). As we apply more personality characteristics to ourself, we become a better person.
As a critical thinker, I like to put my spin on things and come up with my own theories! I think that the sorts of people who play the game are "powerless" and/or "vulnerable" and don't have a strong stand in society i.e. teenagers and maybe the unemployed. People do not like this feeling of being powerless and vulnerable so they create a character that slowly becomes powerful over time. They gain a sense of power as their character in game overcome obstacles such as defeating monsters and completing quests. If the character doesn't feel very strong or powerful, the player is likely to change their characters to something that they believe is more powerful. As the player enjoys this sensation of power and strength, they continue playing so they can keep gaining that sense of strength. I for one like to develop a character from the beginning and look forwards to unlocking the stronger spells, feats and abilities. Yes...my theory is quite controversial and there isn't any evidence, but it's all for the fun of psychology!
What do you guys think? Any thoughts about why you or other people play dungeons and dragons?