The most famous sad friendship poem is Alfred Lord Tennyson's work "In Memoriam," a book-length work he wrote when his friend Arthur Henry Hallam died. Many other poets have also written sad poems about friendship including Robert Frost, W.B. Yeats, Emily Dickinson and even William Shakespeare.
Some of the most famous sections of "In Memoriam" include numbers 105, 11, 22, 27, and 44, and many sections of it can be found online. Some other poems about sad friendship include Dickinson's "The Soul Unto Itself," Frost's "Mending Wall," and Yeat's "Friends." Also, in his sonnets about love, Shakespeare wrote in great detail in Sonnets 1 to 126 about a "fair youth" that seems to be his friend. While many of these poems are happy, 27 to 32 speak of sadness from being apart from him and 78 to 93 are about losing him as a friend.