It's a nice question I always face when I work with parents and teachers.

Many adults think millennials and teenagers are egocentric and narcissists: they take selfies​ all day long, they look at the likes of their Facebook and Instagram, they spend hours when choosing the filter for their pictures.

Actually, it's a short view.

We have to contextualize the behaviour with the age and the times we are living.

When we link the behaviour to the age, we see teenagehood and most of the times the years next to teenagehood are the years when boys and girls build their own Identity.
It starts much earlier, yet the teenagehood is the great rush.

So taking a selfie is not a stand-alone action, but it fits a more complex behaviour: the teen take the selfie, look at the picture, maybe he/she comment on the picture with his/her friends, tests many filters looking for the right one, and only when the teen has the approval of the friends publishes the selfie.
Then he/she waits for the reactions of the online friends and takes feedback from them.
The feedback shapes the future behaviours, and so on.
It's a loop: the selfie is part of the test about the Identity, social feedback shapes this Identity, and so on.

When we link the behaviour to the times we are living, we see the whole loop is the same as choosing a dress.
Simply put, it's the same behaviour acted with modern means such as a smartphone and a social network.

By taking selfies they build their body image too.

The only issue I see when I work with teenagers and sometimes with millennials is the generation gap that is a two-way​ gap: adults think teenagers are too strange to understand, teenagers often are so autonomous with their decisions that forget they are still teenagers.