We are both sixteen, and have been classmates for more than three years. Recently, I have been thinking about her a lot. I've never felt like this before. What's wrong with me?
There is nothing wrong with you. You are just growing up! Becoming an adult is a process that takes time, and many changes take place when you enter adolescence. Some of these changes may make you feel confused or bring conflict into your life.
You will have noticed that many of the things you do or think about (your conversations, readings, thoughts, imagination, feelings, etc) have a boy-girl or sexual theme. This is largely brought about by the physiological and physical changes that have taken place in your body the past few years. The latter is easy to see. For a boy who is now becoming a man, his voice changes and hair appears in different regions of the body. His testes begin to produce sperm, which may lead to nocturnal emissions or "wet dreams" in which excess sperm are released during sleep. For a girl growing up to be a woman, her hips get broader, and breasts develop. Her ovaries start to release ovum, and her menstrual periods begin. Increased activity of the glands on the face could also lead to the scourge of teenagers, pimples!
Another change is the development of your sense of self, or your own personal identity. You become more independent in your thinking, and you do not accept what your parents say to you so readily. You want more freedom and independence to do what you want, and you probably find yourself complaining that your parents are over-protective and restrictive. As you begin to see yourself as an individual, you also begin to see that there are many other individuals around you. Friends become important, and you become sensitive to what they think and how they feel about you. And of course, you now find that you enjoy the company and friendship of the opposite sex. All these changes are part of growing up, and should be treated as natural.