Ponyboys wants to tell Cherry that they're not so different. He may live on "the wrong side of the tracks" and she may be a rich Soc, but they're both teenagers who see the same sunset. They live in the same town and are separated by social class, but Ponyboys wants her to know that that in itself doesn't make them totally different.
On the way home from the movies at the Nightly Double, Pony and Cherry talk a lot and reveal things about themselves that they probably never would have imagined telling a "Soc" or a "greaser" in another situation. They realized how much they had in common, but as soon as Bob pulled up, things had to go back to the way they'd been before.
Later in the book, Pony reminds Cherry that they both see the same sunset.
Ponyboy and Cherry both think on a higher level than any of their friends surrounding them, they both realized that they have so much in common even being a "greaser" or a "soc". Their social class' show the difference between them, they both were able to easily talk with each other when they were alone after the movie they had seen, however when Bob had appeared thing went back to the way there were. Everything came into perspective for Ponyboy as he realized that life isn't as it seems, not all the "Socs" are how they are percieved to by the "greasers". Some of them actually have commonalities that Ponyboy may have. The art of perception plays a big role in this book as many of the charcters are forced to be put inside a imaginary box.
in short to show her that they are the same afterall !