After listening to Wouldn't It Be Nice, I have concluded through scientific evaluation that the song is clearly about death; please allow me to evaluate the song line by line.
'Wouldn't it be nice if we were older Then we wouldn't have to wait so long' This line is almost certainly about wanting to speed up life to a point where death is inevitable through old age and we can see a deep sense of resentment of life through the use of the words 'have to' stressing that life is forced upon us without a choice.
'And wouldn't it be nice to live together In the kind of world where we belong' Anybody with sense can see that this is about longing for an afterlife free from the prejudices of the real world.
'You know its gonna make it that much better When we can say goodnight and stay together ' The 'Goodnight' is clearly a metaphor for suicide with the 'stay together' a reference to the aforementioned afterlife.
'Wouldn't it be nice if we could wake up, in the morning when the day is new and after having spent the day together, hold each other close the whole night through. Happy times together we've been spending, I wish that every kiss was never ending, wouldn't it be nice' I have drawn 2 possible interpretations from this line (and would be interested to know which you agree with). One could see this line as a hope for love to be found in this world however this would be looking at it at face value. A deeper metaphorical meaning emerges when 'wake up' is interpreted as the day after death in which one spends eternity in the never-ending bliss of love.
'Maybe if we think and wish and hope and pray it might come true, baby then there wouldn't be a single thing we couldn’t do' This is referencing the want of a death which isn't just an end and instead a gateway to the previously impossible or maybe even just a want for death itself after becoming apathetic to the real world?
'You know it seems the more we talk about it, it only makes it worse to live without it' An ironic use of the word 'live' given that he is using it in reference to living without death, clearly used for a comedic effect.
I hope having read my completely scientific analysis that you to will agree with this darker, more realistic interpretation of 'Wouldn't It Be Nice'; I can see no error in my logic and therefore believe that my analysis must be correct.