From Longman Dictionary of Contemporary Englishdo something out of the goodness of your heartdo something out of the goodness of your heartto do something out of kindness, not because you have been asked or expect a reward All these people were helping us out of the goodness of their hearts. → heart
Examples from the Corpus
do something out of the goodness of your heart• Surely even an idiot must realise that they wouldn't donate this huge amount out of the goodness of their hearts. 