The first two stanzas of this poem are built from nothing but rhetorical questions—a choice that helps to make the speaker sound cajoling, brusque, and sensible.
The first of these questions gives the poem its title:
Why so pale and wan fond lover?
The speaker's tone in this question is clear from his use of the word "fond," which doesn't mean "affectionate" here, but "foolish." There's a "Why the long face?" tone here: the speaker is making fun of his friend and trying to buck him up at the same time.
That teasing tone gets even clearer when the speaker repeats "Prithee, why so pale?", as he does twice in the first stanza. It's as if his friend has just looked away moodily and the speaker is snapping his fingers under his nose: Come on, seriously, why?
The big central question here, though, is this one:
Will, when looking well can't move her,
Looking ill prevail?
In other words: If she didn't like you when you looked good, why would she like you now that you look like you're wasting away? To this speaker, the performance of unrequited love—whether that means looking "pale and wan" or sounding "dull and mute" (as he complains his friend does in the second stanza)—is nothing but a waste of time. It gets no results from the beloved, it's no fun for anyone else, and it's just plain embarrassing for the mopey lover himself.
The rhetorical questions here thus present the speaker as a guy who knows what he's talking about—and as a friend who's as fed up as he is affectionate.