One can only make assumptions about what leads an author to write. In regards to Thomas De Quincey, those with whom he surrounded himself with most certainly influenced him and helped in his appreciation for both writing and feeling.
De Quincey was friends with other Romantic writers (Samuel Taylor Coleridge and William Wordsworth). Given that he was a Romantic writer, De Quincey was led by his heart. His appreciation for imagination, love of nature and his appreciation of intuition was important given it mirrored the sign of the times for Romantics.
Knowing about De Quincey's talent, some of his friends persuaded him to write Confessions of an English Opium Eater. Given the positive reaction to the text, De Quincey continued to write.