John Keats was getting overly fond of alcohol at an early age. He was often anxious and depressed because he was afraid he would die of tuberculosis, a family affliction. At the beginning of his "Ode to a Nightingale" he is wishing he had a bottle of wine, "a beaker full of the warm south," so that he might get drunk and forget about his own troubles and the world's troubles (“Where but to think is to be full of sorrow and leaden-eyed despair.”) Since he has nothing to drink, he decides he will flee the world in his imagination and join the nightingale which has no troubles since, in his poetic conceit, it is immortal and is the same bird that has been singing the same song since biblical times.
Keats wants to travel to the nightingale's world so he can escape from his own world and hide. Much of his poem is his description of the world he has escaped to in his poetic imagination, beginning with: "Already with thee. Tender is the night." (F. Scott Fitzgerald used "Tender Is The Night" as the title of his best novel.)
Keats had a powerful imagination, which was what made him a great poet. His “Ode to a Nightingale” is an outstanding example of his imaginative powers.
When you're analyzing poetry, it's a good idea to think of the poem as having a speaker who is not the poet.
The speaker in "Ode to a Nightingale" opens the poem by declaring that he would like to experience for himself the source of the nightingale's happiness that inspires it to sing. He isn't content to merely enjoy the beautiful song secondhand.
As the poem develops, the speaker imagines the scenes and sensations that the bird enjoys and expresses his yearning to the bird to "with thee fade away into the forest dim." He envisions that world using the metaphor of drinking an elixir capable of transporting him to a place full of sensory pleasures and providing an escape from the world he finds so demoralizing.
The speaker longs to leave behind the world where he will age with "the weariness, the fever, and the fret" and escape to the bird's place, where these cares are "never known."