Analysis of Sonnet XVIII: Why Art Thou Chang'd?

Why art thou chang'd? O Phaon! tell me why?
Love flies reproach, when passion feels decay;
Or, I would paint the raptures of that day,
When, in sweet converse, mingling sigh with sigh,
I mark'd the graceful languor of thine eye
As on a shady bank entranc'd we lay:
O! Eyes! whose beamy radiance stole away
As stars fade trembling from the burning sky!
Why art thou chang'd? dear source of all my woes!
Though dark my bosom's tint, through ev'ry vein
A ruby tide of purest lustre flows,
Warm'd by thy love, or chill'd by thy disdain;
And yet no bliss this sensate Being knows;
Ah! why is rapture so allied to pain?