Primarily the feeling of discontent with which the reader is left is due to the fact that you know nothing will significantly change. There are multiple instances supporting this idea throughout the story--Randy's speech is the most prominent of these. Essentially, Hinton was suggesting that people will always be just what society expects them to be, that we fit very neatly into the compartments which are created for us. Ponyboy and Sodapop and Darry, while heroic to the reader, are and will remain Greasers to society, just as Randy and Cherry Valance will always be Socs. The economic and social line has been drawn and there is little chance of crossing it. Dally's death at the end is yet another example of how their stations in life can't--won't--change. The boys are proud of who they are and though they have it rough and resent the Socs for their money and status, that pride stands in the way of any potential, remote movement.
(See: Bridging the Gap Between the Rich and the Poor)
Because the ending is a hang cliff. We do not know how the rest of the story goes on.
this is mainly due to your opinion.
Although i also felt sad at the end of the story.To me, it was as if the story was incomplete. The ending could have been happy, but it wasnt. It was the sort of thing that would happen in life. The greasers are greasers. Scos are Socs. Nothing really change. Sure, Ponyboy was in the news paper, but people forgot about that. Life went back to being normal.