I would want to start off by talking about the author's impression of the war from which he was fleeing. Although he was, by his own account, politically naive, at the same time, he had severe questions about the justness of the Vietnam war:
Certain blood was being shed for uncertain reasons. I saw no unity of purpose, no consensus on matters of philosophy or history or law. The very facts were shrouded in uncertainty: Was it a civil war? A War of national liberation or simple aggression?
Such questioning shows the basis for the narrator's decision to try and jump being drafted by fleeing to Canada like so many others.
Then I would refer to the reason why the speaker does not actually cross the border as he planned to do. When he finally has the opportunity to leave the US, he does not take this opportunity, for the following reasons:
I couldn't endure the mockery, or the disgrace, or the patriotic ridicule. Even in my imagination, the shore just twenty yards away, I couldn't make myself be brave. It had nothing to do with morality. Embarrassment, that's all it was.
Note how this leads into the final paragraph of the story, which presents us with something of a paradox:
The day was cloudy. I passed through towns with familiar names, through the pine forests and down to the prairie, and then to Vietnam, where I was a soldier, and then home again. I survived, but it's not a happy ending. I was a coward. I went to the war.
Such quotes are improtant because they show how Tim O'Brien went to war because of the fear of being mocked and disparaged if he chose not to go. However, the end of the story challenges our assumptions by the author calling himself a "coward" because he actually went to the war. Putting these three quotes together, we can see that Tim O'Brien actually considered himself a "coward" because he did not act on his beliefs and allowed himself to be shamed into participating in a war that he was deeply uncertain about.