What's he saying?
"Those lines that I before have writ do lie, / Even those that said I could not love you dearer:"
The previous sonnets I wrote are wrong, especially those in which I said I couldn't love you more than I do:
"Yet then my judgment knew no reason why / My most full flame should afterwards burn clearer."
But when I wrote them, I didn't know that my love would grow stronger.
"But reckoning Time, whose million'd accidents / Creep in 'twixt vows, and change decrees of kings,"
But Time, which brings the unknown and unpredictable, changes lovers' vows and laws,
"Tan sacred beauty, blunt the sharp'st intents, / Divert strong minds to the course of altering things;"
Ruins beauty, dulls human desire, causes people to change their plans based on circumstances;
"Alas! why, fearing of Time's tyranny, / Might I not then say, 'Now I love you best,'"
Back when I wrote those other sonnets, why shouldn't I have said that I love you the most I ever could,
"When I was certain o'er incertainty, / Crowning the present, doubting of the rest?"
When the sureness of my love in the present was stronger than the uncertainties of the future?
"Love is a babe, then might I not say so, / To give full growth to that which still doth grow?"
Love is eternally youthful, so I should not pretend it has reached its limits when it is still growing.
Why is he saying it?
Metaphysical musings about the age of love, and the idea of how it can seem so young and full-grown at the same time, were common around the time this sonnet was written. Sonnet 115 bears resemblance to two poems by John Donne: Love's Growth and Lover's Infiniteness. The last two lines of Love's Growth state, "Me thinks I lied all winter, when I swore / My love was infinite, if spring make it more." The seasons were a common metaphor for youth. In Lover's Infiniteness, Donne ponders of his beloved's heart, "Yet I would not have all yet, / He that hath all can have no more, / And since my love doth every day admit / New growth, thou shouldst have new rewards in store." Though the speaker wants to have "all yet" of the beloved's heart, he acknowledges that he doesn't want to receive it just once, but a little more every day.
Line 5 refers to "reckoning Time," characterizing Time as the thing that keeps track of all human affairs. In other sonnets, Time has been characterized as a destroying, wrecking youth and beauty without mercy. Here, it is still described as a tyrant; in line 9, the speaker admits to "fearing of Time's tyranny." However, this sonnet discusses the unpredictability of time rather than its destruction which is, of course, predictable and unstoppable. This view of time is represented also in Henry IV, Part I, when Hotspur (Henry Percy) says in his dying breath, "But thoughts, the slave of life, and life, Time's fool; / And Time, that takes survey of all the world, / Must have a stop." (1H4.V.4.81-3).
Time is described here as "reckoning," but the poet still addresses the inevitability of its destruction of beauty, a theme he focuses on in other sonnets. Its "million'd accidents," or unpredictable events, "tan sacred beauty." Tanning was the process used to prepare leather hides, and was not a flattering adjective for a person; in Sonnet 62, the poet looks upon his own face, "Beated and chopped with tanned antiquity." Likewise, Time will "blunt the sharp'st intents," or dull sexual desire. The word "blunt" is used in the same way in Sonnet 56, where the poet addresses love itself: "Sweet love, renew thy force; be it not said / Thy edge should blunter be than appetite."
Lines 11-12 address the uncertainty of the future versus the certainty of love. The phrase "certain o'er incertainty" refers to the poet as he was in the past, when he incorrectly declared that he could not possibly love the fair lord more. This phrase is reminiscent of Sonnet 107: "Incertainties now crown themselves assured." The subject of the phrase "crowning the present" is unclear; it could be "my love for you," or "I" of the previous line, or "my certainty." It is also possible that the subject is "incertainty," of the future, which makes it logical to put faith in the present over "the rest."
The final couplet is ambiguous; scholars have interpreted it in contradictory ways. The first, represented in the above section, is that since love is always young, it was wrong of the poet to assert that it was at its height in the past. In giving it "full growth," he was mistaken, since it "still doth grow." However, the opposing view is that "then might I not say so" means, "why shouldn't I be entitled to say...?" Since love is a paradox; it is both eternally young, since it can always grow more, but the poet always feels like that is impossible, like it is full-grown.