Ponyboy, in the early chapters, describes how the various gangs all have sorts of alliances and help each other out. The Shepard gang, for example, might not be the best friends of the informal gang of Curtises and friends. However, if a Shepard needs a place to hide out or crash, the Curtis brothers won't deny that. The boys realize that they who are from the same area must stick together. The Socs are the real enemy, and society has made things difficult for them. Darry leaving the door open is symbolic of the need for all the gangs to be open to each other, willing to help each other - as they will later do during the rumble.