If you’d asked me a month ago I’d have probably said, Eleanor Rigby. (Though I’d never heard it before this, I’m also fond of FlightlessBird’s selection upthread.)
However, I did this thread last month, and find that the suggestion I got in there can’t be topped:
The Intermission track from Christine Lavin’s Live at the Cactus Cafe album.
There is something powerfully appropriate about a track where there’s just some muted sounds of people shuffling chairs.
:smack:
I’m not Thick as a Brick, thank you very much. 
and that was a gutter, as I recall
Si
The Buzzcocks - Ever Fallen in Love (With Someone You Shouldn’t)
I always fall for the ones that are unavailable … the latest one is an international student who goes back to France in August, and its not feasible to follow her.
c’est l’amour, c’est le guerre, c’est la vie…
AP
On a historical basis (including all relationships)? Alanis Morisette. Not the Doctor. Not that I like the song, mind you; I kinda don’t. But it fits.
On a right-now basis? Hüsker Dü, Could You Be The One?. Except not in a depressing way. Or Dido’s Here With Me, which is a special kind of ironic. But not bad.
Dang. That one gets to me as well.
I Fooled Around and Fell in Love.
It’s been sounding more and more like this recently: how to fight loneliness with wilco
“Lonely,
I’m Mr. Lonely,
I’ve got nobody,
to call my own…”
Awesome. Great song, and I get where yer coming from.
“I’m In Love With My Car” by Queen.
Hey, at least my car’s not annoying.
The Stroke by Billy Squire 
Eveyones got to learn Sometime by the Korgis.
Cheesy though it be ours is I got You Babe By Sonny & Cher (among others but that’s the only one I’m willing to share 
I’ve always been partial to Elton John’s “The One”:
We both thought we were doomed to a life alone . . . until we found each other. SO this song gets me every time.
With my recent ex-girlfriend, I’d say Living in Sin by Bon Jovi. Her hardcore baptist family didn’t appreciate her dating an agnostic man.
Now, I’m going to say Cold by Crossfade.
I really like this thread. At various points in my life, I would say these two in the Andrew Lloyd Webber musical, “The Phantom of the Opera.” They are “The Music of the Night”
And “All I Ask of You.”
I hope this link works. http://www.purelyrics.com/index.php?lyrics=hqhxtlsj
And then David Bowie’s theme to Cat People. http://music.yahoo.com/David-Bowie/Cat-People/lyrics/7412190#lyricstop
However, there are many, many others. But these three dominate certain themes in my life. 
‘Lime-Tree Arbour’ by Nick Cave…
Or, for more cynical moments, there’s ‘When You Are Old and Grey’ by Tom Lehrer:
Well after more then 30 years, this is kind of our theme - the fire is still there but in a different way now. I couldn’t imagine life without her.