This post is going to be drawn out, but since it's for your thesis, I don't think you'll care.
The main reason for playing games is for entertainment. It's why we do most anything in life. It's either necessary, which gaming isn't, or it's entertaining. The beauty of games, and movies and books to a lesser extent (in my opinion anyway), is that they can be entertaining for a very large and diverse group of people.
I play games so I can be part of a world that isn't possible for me in my everyday life. I don't play the Sims, because why play someone doing everyday stuff? I want to be part of a fantasy, something I literally cannot do in real life. What separates games from movies and books is the interactivity. No matter how many times you read a book or watch a movie, the same thing happens every time. In a game, that's not the case. Some games have the overall story play out the same way (Call of Duty), but each fight can turn out differently because of the input of the gamer. Other games (Mass Effect) let you actually change the fate of characters in the story.
The main argument I hear about games is that they are mindless and immature. My fiance says this to me all the time. But these are the people who don't take the time to sit down and experience any of them. Gamers are stereotyped just like anyone else. Most people think gamers are nerds or angry. Some people are, but not all. Some people just take it more seriously than others, which is true with everything. I'll be the first to admit, the vast majority of games aren't very good either in story, or controls, or graphics or any combination of the 3. But the same can be said about movies and books. Not every movie is a blockbuster, and not every book in a best seller.
So, why do I play games? Because I get enjoyment out of an interactive experience. I enjoy feeling like I'm making a difference. I enjoy being part of a world I that I wouldn't be able to be part of otherwise. I've played football in the NFL, baseball in the MLB, I was a cowboy in the old west, a human fighting for the universe, a test subject in a lab, an agent fighting off a zombie infestation, a war hero in 3 different decades, an assassin, and a hero fighting dragons, just in the last year. I was able to do this all from my couch, and my experience isn't the same as everyone else who played the same games. That's why gaming is special to me.