Complexity
In official literary lingo, Marvell is what we like to call a Metaphysical Poet. This puts him in the company of some pretty impressive folks like John Donne, Richard Crashaw, George Herbert, and Henry Vaughn (you may have heard of them before) and it also means that Marvell and his metaphysical buddies made names for themselves by digging deep and talking about the "big" questions in life—ones about love, sex, death, existence, the mind, the soul, and so forth. These are pretty weighty topics and, as you might imagine, discussions can easily take a turn for the super-complicated.
Marvell may have been writing a couple decades later than most of the Metaphysical Poets, but he is the King of Complicated, Complex, and Conflicting. The first thing that makes his poetry complex is that Marvell is incredibly elusive as an author. The speakers of many of his poems are often hard to pin down and Marvell uses so many paradoxes, witticisms, and complexities in tone that it's really difficult to tell how he felt about many of the subjects he treated.
On top of all that, Marvell writes about a huge range of topics. He has poems about everything from love and religion to politics and satire, so it's hard to know if you're reading one of his love poems, for example, if there isn't some deeper political meaning lurking under the surface.
"The Garden" is a great example of this; it seems like a straightforward poem about nature, but when you look more closely, religion, politics, and sex start to filter in, too. (Check out "A Dialogue Between the Soul and the Body" or "To His Coy Mistress" for other examples.) So Andrew Marvell, we ask you: Why'd you have to go and make things so complicated? Avril Lavigne, Shmoop, and English students everywhere would really like to know.