out the goodness of (one's) heart
Because of one's altruistic generosity or goodwill, without personal or ulterior motivations. You expect me to believe you would do this out of the goodness of your heart? Come on, what are you looking for in return?
out of the ˌgoodness of your ˈheart
from feelings of kindness, without thinking about what advantage there will be for you: You’re not telling me he offered to lend you the money out of the goodness of his heart?