by choice; willingly; deliberately
Indicates that an action is done out of personal preference or volition, not by obligation or accident. Often used when the subject actively chooses to do something.
See also: 好む
彼は好んで難しい仕事を引き受ける。
He willingly takes on difficult tasks.
Nobody does that kind of thing by choice.
It's not that I'm working overtime because I want to.
No one goes to war by choice.
He eats spicy food because he likes it.
いやいや means doing something reluctantly and against one's wishes, the opposite of 好んで, which means doing something because one likes or prefers it.
Adverbial form of the verb 好む (このむ, 'to like, to prefer'), derived from its て-form. The kanji 好 carries the meaning of 'fondness' or 'liking'.