Why does the poet want to go Innisfree?
Normally one needs to be careful not to attribute the words of a poem to the poet's own feelings because the poet could be writing in the voice of a "persona," a character whose feelings he or she steps into when speaking in the poem. However, we know from William Butler Yeats' own explanation of this poem that the feelings described in the poem were those he actually had at the time he wrote it. Without taking into account that explanation, but only looking at the lines of the poem, we can surmise the poet's motivation.
In the last stanza, he states that when he stands in the roadway or on the gray pavement, he hears lake water lapping. This suggests that he now lives in the city, far away from the beautiful sights of nature described in the first part of the poem. He hears this water lapping "in the deep heart's core." This means that something deep inside him is calling him to return to the country and to the lake where he can experience first-hand the pleasurable sensations he can only imagine while he is in the city.
Although it the island is beautiful, that is not its primary draw. Beyond the beauty of the flowers, birds, and water; beyond the rhythmic and soothing sounds of waves and bees; beyond these sensual pleasures is an emotional experience that the Lake Isle offers him. That experience is peace. Living a solitary life in nature will set his mind and his heart at rest in a way he is unable to achieve in his current urban setting. That is why he wants to go to the Lake Isle of Innisfree.
(In the link below, you can listen to W. B. Yeats explaining how he came to write this poem. It's wonderful!)