How we cite our quotes:
That time of year thou mayst in me behold,
When yellow leaves, or none, or few, do hang
Upon those boughs which shake against the cold,
Bare ruined choirs, where late the sweet birds sang. (1-4)
For good or ill, love tends to get itself wrapped up in appearances. So when the speaker is complaining about becoming increasingly old and crusty he could really be worried that no one will love him anymore, now that he's got gray hair and age spots.
In me thou seest the twilight of such day,
As after sunset fadeth in the west,
Which by and by black night doth take away,
Death's second self, that seals up all in rest. (5-8)
Compare the speaker's self-description here and in quatrain 1. Which would you say is more positive? Yeah, definitely here in quatrain 2. The big difference is that here at least, the speaker makes it sound like he's still got some pep left—that light that's still shining even "after sunset." Even though he doesn't explicitly say so, we could almost see quatrain 2 as forming part of a pick-up line, you know like, "Hey: black night is on its way, and soon it will be time to rest, so let's get busy right now." At any rate, that's more or less what the poem turns into.
In me thou seest the glowing of such fire,
That on the ashes of his youth doth lie, (9-10)
Here we've got one of the same ideas that we got in quatrain 2: the speaker's saying "I've still got it." At least, that's one way of interpreting his reference to the ongoing "glowing" of the fire. Of course, he also says that that glowing is lying "on the ashes of his youth," so he's being honest about how age is creeping up on him. Still, we think the general idea is that he's trying to impress the listener with his continuing vitality.