From Katherine Rundell, Super-Infinite: The Transformations of John Donne, pp. 294-95:
It was very deliberately that he wrote poems that take all your sustained focus to untangle them. The pleasure of reading a Donne poem is akin to that of cracking a locked safe, and he meant it to be so. He demanded hugely of us, and the demands of his poetry are a mirror to that demanding. The poetry stands to ask: why should everything be easy, rhythmical, pleasant? He is at times almost impossible to understand, but, in repayment for your work, he reveals images that stick under your skin until you die. Donne suggests that you look at the world with both more awe and more scepticism: that you weep for it and that you gasp for it. In order to do so, you shake yourself out of cliché and out of the constraints of what the world would sell you. Your love is almost certainly not like a flower, nor a dove. Why would it be? It may be like a pair of compasses. It may be like a flea. His startling timelessness is down to the fact that he had the power of unforeseeability: you don’t see him coming.
The difficulty of Donne’s work had in it a stark moral imperative: pay attention. It was what Donne most demanded of his audience: attention. It was, he knew, the world’s most mercurial resource. The command is in a passage in Donne’s sermon: ‘Now was there ever any man seen to sleep in the cart, between Newgate and Tyburn? Between the prison, and the place of execution, does any man sleep? And we sleep all the way; from the womb to the grave we are never thoroughly awake.’ Awake, is Donne’s cry. Attention, for Donne, was everything. attention paid to our mortality, and to the precise ways in which beauty cuts through us, attention to the softness of skin and the majesty of hands and feet and mouths. . . .
[The quotation is from Donne’s Sermon XXV.]