If we look at the story carefully, we can see that Daniel Webster is a very busy man and has lots of other things he should be doing. In addition, the fact that the case is against the Devil and the vow that Jabez Stone made seems to be self-explanatory without any room for manoeuvre or escape indicates a hopeless case. However, let us see why Daniel Webster himself says he will take on this case and help Jabez Stone:
I've got about seventy-five other things to do and the Missouri Compromise to straighten out, but I'll take your case. For if two New Hampshiremen aren't a match for the devil, we might as well give the country back to the Indians.
Thus we can see that there is something about the character of Daniel Webster that loves a challenge, and the more difficult the challenge, the more he is attracted to it. In addition, there is an element of pride in his response. The way in which he feels that two New Hampshiremen should be more than enough to defeat the devil indicates a tremendous self-belief and strength of purpose that is in keeping with the impression we have formed of his character.