There is little explanation of why Yeats is so hungry for peace in his famous poem "The Lake Isle of Innisfree." Rather, Yeats spends most of the poem describing the pastoral beauty and idyllic peace of a quiet existence. However, one can guess that Yeats wants peace from the trials of urban existence.
Written toward the end of the 19th century, "Innisfree" can be seen as a response to a rapidly changing world. Like the Romantics before him, Yeats appears dissatisfied with conventional existence and yearns to return to an idealized, pastoral lifestyle. Additionally, we can guess that the existence Yeats seeks to escape from is something of an urban, industrialized one. Take, for instance, the poem's final lines:
I hear lake water lapping with low sounds by the shore;
While I stand on the roadway, or on the pavements grey,
I hear it in the deep heart’s core. (10-12)
One of the key lines here is the second one, in which Yeats describes "pavements grey." Though brief, this description hints at an urbanized world, and the description of the "grey" pavements suggests a dull, tedious, altogether tiresome existence. In short, we could say that Yeats is describing a classic modern, urban existence, and so it seems plausible to guess that he is seeking peace from this dreary, urbanized world through the idyllic natural beauty of Innisfree. Though Yeats gives us too little context to be absolutely sure about this idea, it seems to be a sound one based on the information he does give us.