Actually, the person who is singing the song is supposed to be dying of some illness (in fact, the original writer of this song did die of cancer). So he is saying goodbye to the people in his life: his "trusted friend", his girl friend, his father, etc.
He remembers all the good times he had with his friends (we had joy, we had fun, we had seasons in the sun, ...)
He says, it's hard to die "when all the birds are singing in the sky", etc. Meaning, when life is so beautiful, it is very hard to die and leave everything. But he has no choice because of his illness, and he feels very distressed about it.
The song might make more sense to you once you have understood the context in which it is sung.