Deal with Death

Once there was a father and his three loving sons. One day dad met with a serious accident, and Death approached to take the father. The oldest son begs Death to allow his father to live for two more years. Death was kind and agreed. After two years Death return to take the father with him, but now the second son begs for some more time. Death again agrees to another year.
After the year Death returns again to take the father. Now the youngest son requests to allow his father to live just until the candle wick of the candle burning on the bed table burns out. Death agrees. This time however, Death never returns. How come?