How sweet and lovely dost thou make the shame
Which, like a canker in the fragrant rose,
Doth spot the beauty of thy budding name!
O! in what sweets dost thou thy sins enclose.
That tongue that tells the story of thy days,
Making lascivious comments on thy sport,
Cannot dispraise, but in a kind of praise;
Naming thy name blesses an ill report.
O! what a mansion have those vices got
Which for their habitation chose out thee,
Where beauty's veil doth cover every blot
And all things turns to fair that eyes can see!
Take heed, dear heart, of this large privilege;
The hardest knife ill-used doth lose his edge.
But Shakespeare doesn’t trust his own logic, and he warns the boy — his ‘dear heart’ — that the “large privilege” — the free reign he is allowed, may not last forever. If the he pushes thingst too far, the balance will be tipped and his beauty and goodness will — like the rose — be spoilt. 
 There is a note of ambiguity in the address “dear heart”. Is the speaker speaking to himself, acknowledging the incompatibility of the older man and the younger one? His own capacity to be hurt? Or it could be read as a sarcastic address to the boy, with its threatening undertones
Sonnet 95