From the streak, I see that you are still around so I will answer your question with my opinion. If you want to say "he is sad to leave" you would say something such as "El está triste de partir." Notice that this is an emotion so that the verb estar is used.
In the sentence above, the verb ser is used. With ser, you know that a quality is being expressed. It might be easier to understand if the sentence were rearranged to "leaving is sad (partir/salir es triste). That way you can see that it is the concept of leaving that is sad. So, I believe it is the use of ser that lets you know what is being expressed.
That has a very different meaning so prudence is advised when using it.
It is line from the play Romeo and Juliet, by William Shakespeare; Juliet is saying good night to Romeo. Their sorrowful parting is also “sweet” because it makes them think about the next time they will see each other. https://www.dictionary.com/browse/parting-is-such-sweet-sorrow
The verb as used here is intransitive (doesn't have a direct object), and when used in an intransitive fashion it means leave. Also, in English, break sounds a little odd without an object. (My dictionary says partir can mean to break open, as in cracking an egg,, but it doesn't list just break)