Thus can my love excuse the slow offence
Of my dull bearer when from thee I speed:
From where thou art why should I haste me thence?
Till I return, of posting is no need.
O, what excuse will my poor beast then find,
When swift extremity can seem but slow?
Then should I spur, though mounted on the wind;
In winged speed no motion shall I know:
Then can no horse with my desire keep pace;
Therefore desire of perfect'st love being made,
Shall neigh--no dull flesh--in his fiery race;
But love, for love, thus shall excuse my jade;
Since from thee going he went wilful-slow,
Towards thee I'll run, and give him leave to go.

William Shakespeare

http://www.poemhunter.com/poem/sonnet-li-2/