Here are some possible responses to your question:
- So-called “metaphysical” poetry often seems highly learned and intellectual. Donne was himself a highly learned and intellectual man. He had studied at Oxford University (although he was not permitted to take a degree there because in his youth he was a Catholic). He then studied law at London’s Inns of Court. His prose writings are highly learned, and his poetry often reflects a great degree of learning as well.
- “Metaphysical” poetry often deals with a very wide range of topics, often within the same poem. Donne was learned in many different disciplines, including history, theology, philosophy, and the sciences, to name just a few. Donne’s poetry, like his later sermons and other prose writings, reflects his reading in all these different disciplines (and more).
- Most of the other writers who have been labeled “metaphysical poets” – men such as George Herbert, Richard Crashaw, Andrew Marvell, and Henry Vaughan – had strong interests in the Christian religion and often wrote a good deal of explicitly religious poetry. Donne himself had a strong interest in religion (he eventually became a very prominent Anglican priest and a splendid writer and deliverer of sermons). Donne also wrote a good deal of what might be called “religious propaganda.” His poetry, especially his famous Holy Sonnets, reflects his intense concern with religious issues.
- Donne was known in his youth for being a great “visitor of ladies,” and certainly his poetry reflects a strong interest in heterosexual love and in the complications of romantic relationships.
- Donne became more and more overtly interested in religion as he grew older, and his poems reflect this greater and greater interest in religion as well.
- “Metaphysical” poetry is often considered especially “witty” poetry (although not necessarily in the superficial senses of that word), and all the surviving evidence (including his letters) suggests that Donne was a witty man.
- At the same time, Donne could often be a melancholy person (he wrote an extended treatise on suicide), and in the Holy Sonnets, especially, the tone of his poetry is often dark and bleak.
- T. S. Eliot famously praised the “metaphysical” poets for being able to think deeply and feel deeply at the same time. In their poetry, Eliot said, there had not yet arisen what he termed the “dissociation of sensibility” – the idea that poets were emotional and others (such as scientists) were rational. Certainly Donne seems to have been highly capable of simultaneously feeling deeply and thinking deeply. His poetry is as interesting for the ideas it explores as for the emotions it expresses.
For a biography of Donne deliberately designed to link his life with his writings, see George A. E. Parfitt, John Donne: A Literary Life (New York: Palgrave Macmillan, 1989).