Well, I have two ideas:
1.) This is not sadness, but the seriousness and fundamental nature of the feeling that engulfs young hearts. This feeling (read love) so saturates a person with a certain depth and comprehensively satisfies that there is no longer a desire to try to achieve some unnecessary emotions or situations at the moment (giggling over jokes, sabotage over empty topics, etc.)
2) They are just sad in anticipation of parting. Moreover, the first option for parting is forced for some domestic reasons. For example, young people live with their parents and cannot normally and for a long time retire, which is what all lovers want so much. And so, for example, they are standing at a bus stop and waiting for a bus that will separate them for a few more hours / days / weeks / months. Yes, this reason is not as romantic and pathetic as the previous one, but from this it is no less vital. Another option is that they are waiting for the parting of the heart, not everyday life. And about the reason for the heartbreak of lovers / lovers, you can write a separate book. What reasons can come to mind for the young Romeo and Juliet of the modern world! Example: they love each other, but feel that for some reason they are badly influencing the other partner, and therefore the best option for development would be to stop contact. Or it is corny one of the partners already feels: love has passed, tomatoes have withered, sex and tenderness are not the same and everything is not the same.
The faces are not sad, the faces are saturated with seriousness and a certain trait of love.
A certain impulse is transmitted through hugs and everything is overflowing in the soul. I don’t know what other expression on my face with such a strong feeling of being human. You feel your beloved (oh) and understand the seriousness of your love.