The epigraph at the beginning of Samuel Johnson's poem "London" is a quotation from the Roman poet Juvenal. It reads, "Quis ineptae / Tam patiens urbis, tam ferreus ut teneat se?" Translated, this is, "Who can endure this monstrous city? Who is so iron-willed he can bear it?" This set of questions sets the tone and the theme for the entire poem.
The speaker would likely answer these questions with a firm "No one!" This is why he is leaving London. He can no longer endure its filth and poverty, its crime and corruption. It is filled with decay and malice, appalling ideas and contempt. The speaker wants to "find some happier place" where there is honesty and peace, a quiet rest in nature, and room to think clearly and relax. London is no longer that place, and the speaker lacks the iron will that can bear the monstrous city.
Further, Johnson composes his poem in imitation of Juvenal's "Third Satire," so it is fitting that his epigraph is a quote from that poet. Johnson wants to capture the corruption of London through the tool of satire, the use of irony, ridicule, exaggeration, and humor to critique the folly and degradation of human life and society.
Johnson actually goes even further in his satire than Juvenal, for Johnson includes politics in his poem. He alludes to such contemporary matters as pensions, licensing laws, and the abuses of the ruling government as part of his rant. These things contribute to the horrors of the monstrous city, and the speaker cannot wait to get away from them all.