The Ultimate Love Song... for me anyway:
"I Don't Know Why" by Shawn Colvin
"I don't know why
the sky is so blue.
And I don't know why
I'm so in love with you.
If there were no music
then I would not get through.
I don't know why
I know these things but I do. I do."
It is a simple song, guitar accompanyment only, that just grabs you.
I wish it were legal to post songs on this but since we can't, I guess you'll just have to take my word for it.