What two questions about "motives" does pope want the Muse to answer?
The first question that Pope asks the muse to answer explicitly involves motive:
Say what strange motive, Goddess! could compel
A well-bred Lord t' assault a gentle Belle?
The second question is essentially an inversion of the first:
...what stranger cause, yet unexplor'd,
Could make a gentle Belle reject a Lord?
Obviously, the poem is an attempt to answer these questions through what is best described as a "mock epic." The invocation of the muse was a common device used by epic poets since the days of Homer, and by using it in this poem, he gives the subject matter a bit of gravitas. By describing a (real life) event that really isn't all that important in the grand scheme of things in epic terms, Pope pokes fun at the vanity and self-absorbed nature of many of his fellow Englishmen. The form, more than the content, of the introduction is part of the satire.