心甘情愿 ------- of one's own accord

例句：
A: How come you helped him do so many things without asking for any reward?
为何你帮助他做如此多事，而没有要求任何报酬？
B: I owed him some favors, so I did so of my own accord.
我欠他一些人情，因此我心甘情愿如此做。

同义词：
on one's initiative of free will