The subject of this film is human obsession with happiness that someone else should bring to them. It is usually called a need for love. This is a story about the states that people get in when that love is not returned.
Happiness in love is not just a phrase; it has a foundation in physiology. People in love increasingly secrete endorphin, or "the hormone of happiness", through time, the secreting decreases and when it eventually stops, i.e. when one comes back to "normal", life seems completely futile and meaningless. Today these states are called emotional addiction. That is why "getting off love" is as painful as "getting off" nicotine or some other drug. It is no wonder that some more conservative thinkers treated those conditions as illness.
Then again, who has not been "ill" at least once? Most of us would give everything to "fall ill" at least once again.