In the late 18th century, sentimental friendships were all the rage. These were and are extremely close, very emotional friendships in which the two people do not hesitate to declare their undying love to one another, spill out their innermost secrets, hang around each others' necks whilst weeping, and kiss each other. The Amateur Historian theorizies that this was particularly popular amongst the upper classes in the late 18th century, where most marriages were not based on love, and there was no divorce if love-matches sadly went sour.
Your average bourgoisie or aristocrat would therefore have to find the emotional fulfillment necessary to a happy existance outside of marriage. This resulted either in affairs (which were generally accepted if the couple in question were discreet) or in sentimental friendships.
The poet Samuel Taylor Coleridge had a disappointing marriage (in The Eolian Harp, he begins to wax poetic about his wife, who promptly tells him to shut up) and developed a sentimental friendship not only with William Wordsworth, but with Thomas Poole. Unfortunately for the increasingly dependent Wordsworth, during their trip to Germany, Coleridge realized that Poole was more necessary for his emotional well-being.
Coleridge's letters to Poole include: "The Ocean is between us & I feel how much I love you!", "Of many friends, whom I love and esteem, my head & heart have ever chosen you as the Friend--as the one being, in whom is involved the full & whole sacred title....", and "My spirit is more feminine than your's--I cannot write to you without tears/ and I know that when you read my letters, and when you talk of me, you must often 'compound with misty eyes'...."
The Amateur Historian's personal favorite?
"O my God! how I long to be at home--My whole Being so yearns after you, that when I think of the moment of our meeting, I catch the fasion of German joy, rush into your arms, and embrace you.... Now the Spring comes, the vital sap of my affections rises, as in a tree!"