Jephthah came to Mizpah to his house; and, behold, his daughter came out to meet him with tambourines and with dances: and she was his only child; besides her he had neither son nor daughter.
The grief of Jephthah was not because his daughter would become a burnt-offering, but because ... as she was his only child, it was the end of his posterity upon the earth.  In the thinking of the people of his day, this was more than enough to break his heart.