Songs are generally titled based on what is repeated in the chorus, or at least something that is mentioned in the song that is key. But there are plenty of songs that have titles that, while not mentioned explicitly in the song, are essential to understanding the song properly. Take The Velvet Underground’s “Venus in Furs”. Nowhere in the song is the title said, but is implied everywhere in the myriad references to Sacher-Masoch’s tale. On the other hand, the title could be complete nonsense or at least separate from the nature of the song. For instance, Stereolab’s “The Noise of Carpet”…nowhere is that actually mentioned in the song, but it does add an intriguing ambiguity. A few more examples:
“Banquet” - Bloc Party
“Tomorrow Never Knows” - The Beatles
“Alone, Together” - The Strokes
“Blue Monday” - New Order
“Baba O’ Riley” - The Who
“Primary” - The Cure
“Good Song” - Blur
“Clint Eastwood” - Gorillaz
“How I Wrote ‘Elastic Man’” - The Fall (he clearly says plastic man, instead…*rolls eyes*)

Radiohead did this extensively on Hail to the Thief with all of the tracks having a parenthetical subtitle not actually mentioned in the song, such as “The Gloaming. (Softly Open Our Mouths in the Cold.)” and “A Wolf at the Door. (It Girl. Rag Doll.)”. While some may perceive this as gimmicky, I think it is more likely that such additional words in a title are likely to encourage people to actually listen closer, even going farther to develop their own theories about what the meaning behind the song could be (see this page for more on this).

Of course, most songs *are* labeled quite clearly. Clear labeling of a song can allow a radio-listener to figure out what that great song they heard was with relative ease, and it is just more obvious, and perhaps less pretentious to do so anyhow. Song titles can instantly tell person what the song will probably be about. Then, of course, instrumental songs always have song titles that set the stage for their interpretation, because there are no vocals to clue you in. This is a time where the title is even more crucial, and can make or break people’s interest in even hearing the song in the first place. When I first had put my music up on Last.fm, my most popular song was “Tiger Lotus”, and I attribute it more to the title than the sound of the song itself!
….This just happens to be a topic that interests me currently, as irrelevant as it is.