Why do people turn into emo?

People don't turn 'emo'. Emo is a stereotype.

If you mean someone who causes harm to themselves, rather by cutting or another method, it may be their way of dealing with stress or pain.

People may turn 'emo' because of family problems, school, peers, or other issues.

If you mean the people who wear a lot of black, eyeliner, and plaid/etc. Then that is just a style of fashion. Emo is basically the mix of Scene {teased hair, too much make-up, funky clothes] and Goth [wear all black], it's just because they want to.