Interesting quote. I can agree with Shelley in a certain sense, but for the most part I disagree. I think that you can sit down and decide to write a poem, in the same way you can sit down and decide to write anything else. Part of poetry is playing around with the sounds of words, and in that sense it is very possible to deliberately write something that plays with sound, meter, and rhyme. And even if playing with words isnâ€™t your primary goal for writing a poem, or if you write open form poetry instead of more traditional poetry, I do think itâ€™s possible to make yourself write a poem. Most writers who actually write stuff are the ones who discipline themselves to make themselves write, because if you wait for a moment of inspiration, you might possibly be waiting forever.
Shelley was a Romantic poet, so I think the reason he says this sort of thing is because the Romantic movement placed great emphasis on emotion, and if you want to capture an emotion, or the feel of a certain moment, that is definitely something that is best done in the heat of the moment, without waiting. Thatâ€™s the sense in which I would agree with him.
So, in short, I agree in that there are certain thingsâ€”emotions, experiences, flashes of inspirationâ€”that do need to be captured right away (and then can be revised at leisure); however, I think that not all poetry is necessarily like that (for example, Iâ€™ve written poetry for assignments before, and so have many other people), and that it is possible to write poetry according to a determination of the will. One has to take these Romantic ideas with a grain of salt.