Why doesn't the speaker think he will ever have a chance to walk the other road?
If the poem is taken literally, then the speaker would be able to go back and take the other road. However, this notion of choosing one of two roads is a metaphor for choosing one of two (or more) paths in life. In the third stanza, he says (with hope but knowing it is an impossibility), "O, I kept the first for another day!" He follows this thought with the reality that, "Yet knowing how way leads on to way, / I doubted if I should ever come back." The idea is that when we make certain choices in life, it can become impossible to return and take the other path. For instance, if the speaker were to choose to travel the world rather than settle down and marry, he might decide years later that he would rather be married. But in the meantime, the person he considered marrying is now married to someone else. Therefore, that road is now gone. He cannot go back and take that other road.
In the final stanza, the speaker claims that he took the road less traveled by. But he is purposefully kidding himself. He says this with a "sigh" - he foresees a time when he is older, thinking back on his decision. He foresees his older self trying to convince himself that he took the road less traveled, the road most people would not take. But both roads seemed to be traveled about the same: "Though as for that the passing there / Had worn them really about the same." He wonders where the other path might have taken him. But he kids himself in thinking that he took the road less traveled; both roads looked the same, or as good. Both choices seemed to have their merits, but he could only pick one. And in picking one, there is the possibility that he might never be able to return to take the other.