The old word fain describes the feeling of gladly or willingly doing something. You would fain have stayed home, but had to venture out in the rain.
The word fain comes from the Old English fægen meaning "happy, well pleased." When you're fain to do something, you are happy to do it. I'd be fain to pick up a twenty dollar bill I found on the street. Sometimes, though, the word is also used when you're compelled by obligation to do something. If I saw that dollar bill fall out of a man's pocket, I would be fain to flag him down and return it.
