The old-fashioned word fain describes the feeling of gladly or willingly doing something. You would fain have stayed home, but you had to venture out in the rain.
Fain comes from the Old English fægen, meaning "happy, glad, or well pleased." When you're fain to do something, you are happy to do it, like when you're exhausted and would fain take a nap. Sometimes fain is also used as an adjective when you're obligated to do something, or when you're happy enough, given the lack of better options: "He was fain to hand over half the lemonade stand money to his little brother."