FAQ for
Elephant (2003) More at IMDbPro »

Its because neither of them had kissed anyone before and so wanted it to experience it before they died. If you listen to their dialogue during the scene they actually mention that they could die and they haven't kissed anyone.

You see Nathan and Carrie run to the back of the cafeteria (to the freezer) when they see Alex walking down the hall. Alex walks into the cafeteria appearing to not notice that Carrie and Nathan were in there just a few moments ago, sits down at a table and takes a drink from a cup sitting on the table. Eric comes in and briefly talks to Alex about the casualties that he's taken, but is then shot mid-sentence by Alex. Alex walks out of the cafeteria, showing little emotion over shooting Eric, towards the freezer. He opens the freezer and discovers Nathan and Carrie then begins to recite eeny, meeny, miny, moe to determine who gets shot first; As he ends the rhyme, it cuts to the cloudy blue sky similar to the one at the beginning of the film, first with ambient music then shortly followed by Fur Elise, the same song played on the piano by Alex earlier in the film.

The movie never does show who gets shot, the film ends with no resolution, making it up to the viewer to decide who gets shot; whether he shoots Carrie or Nathan or, perhaps, himself.