The instrumentation can also contribute to making an OST suitable for a sad love story. Soft instruments like the piano or the violin are often used. The piano can create a very intimate and sad atmosphere with its solo notes. The violin, with its ability to produce long, drawn - out, and sometimes screechy notes, can add to the sense of drama and sadness. Also, the use of ambient sounds or a minimalistic approach in the music can enhance the feeling of emptiness or loss that is common in sad love stories.
The overall mood and the way the instruments are played. Maybe the story it conveys through the music is one of separation and longing, which gives it that sad love story feel.
I don't think so. There haven't been any official announcements or indications that the HxH manga will continue. But you never know, maybe there'll be a surprise in the future.