Ode to a Nightingale

I'm interested in the phrase: "Thou was not born for death, immortal Bird!" I don't really know what to make of it. I can't decide if he is saying that animals are immortal because they lack consciences and thus have despairs because "where but to think is to be full of sorrow." I would think that because he aspires to be like the nightingale, he would simply want to tap into his more spiritual and natural side through poetry, rather than lose his conscience--so to speak.

Nature plays a strange role in this poem. As I already said, he both aspires toward nature as well as derogates its to an extent. Also, at the beginning he metaphorically speaks of plants like hemlock and opiate that have caused him to have a "drowsy numbness" that pains his sense. He then goes on to talk about plants that have "soft incense" and sweetness. I really can't understand what he thinks of nature in this poem, but nature seems to be the crux of this poem. I think it is almost supposed to be both something beautiful and something deleterious, but it is very confusing to me. If anyone has any opinions on nature in this poem, I would really like to hear them.