Is my betta depressed?

I've had my betta about four months and I love him (as much as you can love a fish, that is...). I'm a writer and my books and notebooks are everywhere in my room, so I moved his tank into the den so I can have a bookshelf. No one's in the den very much and when I come in to check on him, he seems really sad and mopey. Usually he swims all over the place and loves to show off his colors and is very active. He would never hide or anything in the plants, but now that's all he wants to do and is very still until I'm watching him. I wonder if it's from lack of interaction with humans? He doesn't have ick or any parasites on his body (that I can tell), his fins are fine, eating fine, but even my mom says he seems sad. Can bettas be depressed, even? Should I move him back or is he sick?