Desire, Action and Conflict - The essence of story

Why did I find the incident mentioned in the previous post a germ of the story?

Because it had action. And by action, I don't mean something like a person walking on the road or a bird flying. It has a boy fleeing from his pursuer. It has a police trying to catch an offender. You see a conflict of interests. The action of the boy hawking vegetables in train (an illegal thing) has put him in conflict with the police. The desire of the police to enforce the law has put him in conflict with the boy.

So that is the essence of story.
1)Your protagonists should want something. Anything. -Desire.
2)Your protagonists should try to realize their dream by working towards it. -Action
3)His action should put him in confrontation with another entity (another character) - Conflict.

Now it is up to you as an author to decide how to keep escalating the conflict through the middle of the story and then finally resolve it by the end of the story.
You let your protagonist win in his mission - happy ending.
You make him loose - sad ending.
You maintain a status quo - ambivalent ending. (By the way, this is not preferred. You should at least convince the reader that he has come out wiser of the experience. Who likes status quo in a story. They have plenty of it in real life already.)