Reason : The definition of "Goodness" is "the quality of being morally good or virtuous." Watermelons are sweet, refreshing, nutritious. Therefore, the watermelon is used to symbolize "Goodness" as a reminder to always be sweet and a light to those we encounter on our day to day lives, even when it's not always easy.
Butthe fruit of the Spirit islove, joy, peace, patience,kindness, goodness, faithfulness,gentleness,self-control;against such things there is no law.