Is "happiness" or "unhappiness" a more appropriate theme for "Ode to a Nightingale"? We're not sure. The speaker is mighty unhappy about the demands placed on him by life, time, and age. He hates to consider that young, beautiful people – the Romantic A-list – will eventually be old and incoherent. But he claims that the "ache" his heart feels is due to extreme happiness for the nightingale, so we'll have to take his word for it. He seems content enough, at least, to have an "I could die happy" moment around the middle of the poem.
The speaker is jealous of the nightingale's immortality. His so-called "happiness" is just self-pity.
The poem argues that we cannot create our own happiness; it is dependent on the world around us.