It's also well known for being ambiguous. Birds are in the forest, fish in the water, and I have to go crazy. I walk in great sorrow for the best (a beast?) of bone and blood.

Average guess seems to be that it's the usual springtime theme: All of nature's happy but me, because she doesn't love me.

However, it's also been interpreted as a hatred of the flesh: Fish and birds are where they're supposed to be, but here I have to go crazy for the goddamned mortal body of a human animal.

And it's ALSO been interpreted as a Christian lyric, of the kind where male/female love is a metaphor for love of Christ: Fish and birds are following their natures, and similarly I must go crazy for the best creature that ever lived (beautiful woman, or the incarnate Christ who himself walked with the greatest sorrow...)

Hey, if it weren't for this sorta stuff, how could English majors EVER make a living?