That's not new teens often have conflicts inside their mind and they really don't know what to do with their problems. They don't have neither knowledge and skills to solve them nor people who could support 'cause they don't want to trust them. As the result, we recieve nervious and depressed teenagers who already don't have energy to deal with problems. But the thing is the pressure is too strong so they usually can't keep everything inside. Because of that, teens find someone to tell about their problems, most of the times their friends play the role of great listeners.
That's a good way to solve the problems 'cause teens can understand each other easier than teens and parents, for example. Also sharing worries makes the friendship stronger, that means two people get closer to each other what affects positively on mental health - everybody loves to say, a friend in need is a friend indeed. But it has its own consequences.
The difficulties start when a "listener" gets too much pressure. When we listen about something, our level of attention depends on how close is it to us. More details which can be compared with our situation - stronger our feelings are. So, when our friend shares a problem with us, we automaticaly start to think: "What if it's happening with me too?" That's how our mind works - the system of monitoring the danger is always on, and because of that state, the listener's anxiety rises. That leads to stress, and we get two nervious teens. So the help by listening and talking isn't always the best way to solve the problems in case it's not only harmful for the side who is trying to help but and for the teen with a problem as he doesn't find any efficient support. I'm not trying to say that listening to your friend when he has a tough period in his life mustn't be used, but you have to be attentive and care about your state firstly. If you understand you are suffering and you can't continue helping, it'll be better to don't let your friend make you feel bad and ask an adult for an advice - this way, you both won't get more problems trying to support each other. Sometimes we need help, but it has to be safe for everyone.