Sappiness is the quality of being overly emotional or sentimental.
Sappiness is often seen as a negative quality because it can be too much emotion.
Examples:
The sappiness of the movie made her cry.
I don't like the sappiness of this song.
The sappiness in the story was too much for me.
The sappiness of the love letter made everyone laugh.
The sappiness of the movie was so strong, it made me feel sick.
The sappiness of the movie was like a big, wet hug.