Why do men become aroused?
Arousal is the first step in your body's preparations for sexual intercourse. Both boys and girls experience arousal after they past puberty, but arousals are more noticeable to boys since it primarily involves having something that feels like a tent pole in your pants.
When you first gain the ability to have full-blown erection (one where your penis increases size as well as becoming stiff), your mind is inexperienced with the new connections. As a result, you have erections at odd times for no apparent reason. These are called spontaneous erections. It takes the brain a year or two to get the hang of limiting erections to situations that the mind perceives as sexual in nature.
Unfortunately, there is a period of time where nervousness can trigger erections. Hence, when you are giving a report in front of the class, your penis may become erect. Of course, then you become concerned that someone might notice and that nervousness just strengthens the erection. A similar thing happens when you have to take showers at school. You're nervous about being naked in front of others and that nervousness triggers an erection. Then you get teased about being "turned on" by naked boys and the seed of doubt is planted. You begin to wonder if it is really true. I wish I could tell every boy that what is happening is perfectly normal. It is the way your body is supposed to work.
"Why," you asked? One of these days you will marry and then you will want to have sex. But sexual intercourse involves a number of different parts in your body that must act together for sex to take place. Imagine doing a play where you never practiced or even read the script before opening night. Imagine getting a job as a trapeze artist, but your first time up is in front of a paying audience. We all realize that for something to work well, you need practice. Arousal and wet dreams are the ways your body practice for eventual sexual intercourse in marriage. It is annoying when your body gets its signals crossed early on, doing things at the wrong time, but with practice it gets better.