The young man's outward beauty is universally praised, yet when people examine his inner character through his actions, they find his reputation contradicted—his beauty suggests virtue, but his behavior hints at base nature ('thou dost common grow').
Sonnet 69's central mechanism is the distinction between 'outward' and 'mind,' between what the 'eye hath shown' and what is 'farther' than the eye can see. The sonnet's first half unifies appearance and judgment—the world's praise matches the world's vision. Yet the volta introduces a psychologically devastating reversal: those very people who praise externally simultaneously form different judgments internally. Their 'thoughts' become 'churls' (base, lowborn), contradicting their 'eyes' which 'were kind.' This split consciousness—kind eyes, churlish thoughts—describes the experience of seeing beauty undermined by behavior. The young man's appearance remains perfect ('thy fair flower'), but his deeds introduce discord into perception, forcing the speaker and others to hold contradictory assessments simultaneously.
The metaphor of odor—'thy odour matcheth not thy show'—is particularly effective because smell is involuntary and visceral. One cannot choose what to smell the way one can choose what to praise aloud. The image suggests that beneath the visual beauty (flower) lies something base and corrupting (rank weeds). Yet the couplet's diagnosis—'thou dost common grow'—is ambiguous. Does 'common' mean he is becoming common, or does it mean he is frequenting common places and people? The suggestion of promiscuity or degradation lurks beneath the surface. His beauty remains transcendent, but something essential—his character, his behavior, his essence—has become debased, 'common,' ordinary. This loss of aristocratic distinction is more devastating to reputation than physical aging would be.
Like when someone is stunningly beautiful but then you discover through their actions that they're mean, vain, or shallow. People overlook a lot because of looks, but once behavior is known, beauty becomes complicated. He's gorgeous but 'common.'