The speaker is gripped by pathological self-love, seeing himself as superior in all ways until seeing his reflection—aged and worn—shatters this illusion, revealing his self-love actually masks his love for the young man.
The sonnet's first octave presents self-love not as vanity but as a metaphysical state: the speaker cannot escape the self-love that 'possesseth' him entirely. It is 'sin' because it corrupts perception—the speaker's 'glass' does not truthfully reflect his aged, weathered face. The shock of the mirror in line 10 ('beated and chopt with tanned antiquity') is devastating because it shatters the false perception. Yet rather than cure the self-love, the mirror redirects it: the speaker cannot escape loving himself, but he transfers that love to the young man. The 'myself' of line 13 becomes the beloved, a metaphysical collapse where the speaker and young man are unified. Narcissism is transformed through love into merger.
The couplet performs a stunning inversion. Instead of condemning narcissism, it redeems it by collapsing the boundary between self and other. 'Tis thee, myself'—the young man is the speaker's true self, so loving him is not self-love but the highest form of truth. The speaker 'paints' his age with the young man's youth, using the beloved as a kind of mirror to restore what time has stolen. This is not degradation but a form of metaphysical optimism: the speaker can love himself truly only by loving the young man, for the young man represents what the speaker's true self is—beautiful, young, worthy. Self-love and other-love collapse into one.
Like someone obsessed with their own image on Instagram until they accidentally see their candid photo and are horrified. But then they realize their obsession wasn't actually about themselves—it was about wanting to be as beautiful as someone else they love.