https://www.quora.com/What-is-a-good-analysis-of-Sonnet-77-by-Shakespeare
Thy glass will show thee how thy beauties wear,
Thy dial how thy precious minutes waste;
The vacant leaves thy mind’s imprint will bear,
And of this book this learning mayst thou taste.
The wrinkles which thy glass will truly show
Of mouthed graves will give thee memory;
Thou by thy dial’s shady stealth mayst know
Time’s thievish progress to eternity.
Look, what thy memory cannot contain
Commit to these waste blanks, and thou shalt find
Those children nurs’d, deliver’d from thy brain,
To take a new acquaintance of thy mind.
These offices, so oft as thou wilt look,
Shall profit thee and much enrich thy book.
I like the phrase "mouthed graves." The suggestion seems to be that the serious thoughts we voice are reflected in our faces and when we look at ourselves, our wrinkles remind us of those sad times. Shakespeare is encouraging a kind of bibliotherapy here. Our most serious thoughts, brought about by our most trying times, are those that will most enrich our "book."
The analysis in this link, however, doesn't go into this, but is a very thorough analysis of the poem.
