My prior vows of love were lies because time can change all things, even sacred devotion; I should have hedged with 'now' rather than claiming eternal constancy.
Shakespeare catalogs time's destructive powers: 'millioned accidents' that 'creep in 'twixt vows'; time changes 'decrees of kings'; it tans sacred beauty and blunts sharp intentions. This is time as erosive chaos, not the majestic Time of later sonnets. Every human commitment is subordinate to temporal accident. The extended meditation on time's power establishes the preoccupation that will dominate sonnets 115–126.
The couplet redefines love as 'a babe' that still grows, justifying the speaker's refusal to have made eternal claims. Love is not a fixed state but a living thing in flux. This rhetorical move—claiming youth and growth as excuses—will be complicated by later sonnets that insist love must be eternal precisely because time destroys all else.
You once promised 'forever' to someone and meant it completely. Years later, you realize forever is impossible—not because you're flaky, but because people change, circumstances shift, and time always wins. You're acknowledging that your past certainty was naive.