There are several reasons and causes why people get addicted to the internet; boredom, loneliness, online TV, social networks, etc. They could be lonely in the way that they're the only child, or they have no friends due to being home schooled, so they waste their time on the internet. Or they could just be bored; they could be home alone, or it could be raining outside, and they can't go out, so they stay in and surf the web. There are also a lot of social networks online like chat rooms, Facebook, Twitter, MSN, and more. People enter social networks to meet new people, and get into virtual online relationships. Entering social networks can also relate to loneliness sometimes.
Spending too much time on the internet can affect you very much. It can interfere with your social life, the addiction can lead to the inability to communicate in the real world, and your family bonding. You could have mixed feelings of guilt while at the computer, and it can also distract you from your work/school related work. And because of the lack of exercise and movement, obesity is a huge symptom. To some people, when you are not on the computer, you think about it frequently and anticipate when you will use it again.
I suppose that if anyone is too addicted to the internet, they should maybe set an alarm clock for only 1-2 hours, to lessen their addiction. Or they could talk to their mothers, or guardians about it, or maybe get a therapist for some help, and try their best to think of all of the effects that could happen to them, and try to stop.