Here the poet uses a perhaps surprising argument - he says that the fact that he's ageing, reaching the autumn of his years, should make the friend love him more, since he will have to part with him soon. The image of the 'bare ruin'd choirs' is a reminder that in the England of the Reformation there would have been so many ruined monasteries dotting the landscape.
That time of year thou mayst in me behold (Sonnet 73)
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 ruin’d choirs, where late the sweet birds sang.
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.
In me thou see’st the glowing of such fire
That on the ashes of his youth doth lie,
As the death-bed whereon it must expire
Consumed with that which it was nourish’d by.
This thou perceivest, which makes thy love more strong,
To love that well which thou must leave ere long.