What is it about some girls? You know those girls, the ones that become some attainable ideal of perfection to their many admirers, one of whom is the most pure of spirit and intention. This one person who admires the girl from afar and waits until the perfect moment to have his chance with her. This inevitable moment will be wrought with tension, then the girl will see that the admirer is the one for her, and they ride off into the sunset because it's twue wuv. Or at least that's the ideal. What's so much more special about these girls? If there's anything, I don't think the admirer really knows. Idealizing someone from afar means you can never really know the person. Therefore, you aren't in love with the girl, you are in love with your own idea of the girl. You're also probably kind of creepy, but that's a different story.
One such girl is Margo Roth Spiegelman. Quentin Jacobsen loved her since they were children together. Paper Towns opens with little Margo and Quentin discovering a dead body together. Then there is a flash forward to their senior year of high school. Margo and Quentin lost touch a long time ago. They no longer belong to the same social circles. Margo is popular, while Q hangs out with the band kids, despite not even being in the band.
One night, without any warning, Margo climbs into Quentin's bedroom window. She recruits him on a mission to get revenge, "right some wrongs and wrong some rights." At first Quentin resists, but soon he is pulled into the plan and he has the night of his life. He doesn't know what to expect the next day at school, but it turns out that Margo doesn't show up. She's gone. It was normal for Margo to disappear for days or weeks at a time and leave cryptic clues for her parents to follow. This time, though, it seems as though the clues are for Quentin.
He follows the trail with his friends Radar and Ben, and with Margo's best friend Lacie. Woodie Guthrie leads to Walt Whitman leads to an abandoned storefront leads to paper towns, but they are no closer to finding Margo. Soon enough, they start to wonder (I actually thought they were a little slow in this because I wondered about this much earlier) if they will actually find her alive, thinking about things she said about cutting all her strings, misinterpreting Leaves of Grass as a suicide note, and finding a graffitied note, "YOU WILL GO TO THE PAPER TOWNS AND YOU WILL NEVER COME BACK."
Quentin realizes in his searching that he never really knew Margo at all. He knew his idea of her as this perfect girl, the girl he loved all his life. He knew the legend of Margo, but he didn't really know Margo. There was a point when Quentin asks his English teacher for help interpreting Whitman's "Song of Myself" and he admits to looking for Margo when reading the poem instead of actually reading the poem to understand Whitman. In the same way, Quentin uses the clues to understand Margo, but his idea of Margo, not the actual Margo.
...You shall no longer take things at second or third hand, nor look through the eyes of the dead, nor feed on the spectres in books,
You shall not look through my eyes either, nor take things from me,
You shall listen to all sides and filter them from your self.
Walt Whitman, "Song of Myself"
So nobody really knows anybody. Everybody is connected through blades of grass or invisible strings, but nobody really knows anyone. Someone you (think you) know could have a secret record collection or be an axe murderer. You can fall in love with someone and then fall in love again when you really know who that person is. Life is entirely too much about the future and not enough about the now. Finally, when you're on your death bed, you will inevitably be grateful that you carped at least one diem. So go and do that soon.