Some explanation here
http://en.wikipedia.org/wiki/MacArthur_Park_(s ong)
I have always thought of it as a reflective song, maybe a young man sitting in Macarthur park wondering why he has broken up with the "love of his life", and will he ever find anyone else.
I always think the section that starts "Someone left the cake out in the rain" is about the fact he thought he and his girlfriend would be together for ever, but then it all went sour and they broke up and he will never find such a good relationship again (when he says "never find the recipe again").
I think the second half may be the same person as an older man, again looking back at his life, thinking about all the girls he went out with and the "special" one he lost.
I always find this section very moving when he sings it.
And after all the loves of my life
After all the loves of my life
I'll be thinking of you
and wondering why.
I do love the song, and the arrangement, but it can be rather pretentious in places.