And both that morning equally lay In leaves no step had trodden black.
Here, again, we hear that the paths are equal, but we find out something new, that it's morning. It's possible that our speaker is the first to travel to this place on that day.
The paths are covered with leaves, which haven't been turned black by steps crushing them.
Wait, we thought one path was grassy…and now it's covered with leaves. Possibly, the leaves aren't very thick, or the grass sticks up in between them. Or maybe the speaker isn't being quite honest.
Oh, I kept the first for another day!
The speaker seems like he's already regretting his decision. He is rationalizing his choice of path by saying he'll come back to the one he missed later.
This is a familiar way to deal with difficult choices; "you can always come back and try it again later," we think.
With an "Oh" at the beginning and an exclamation point at the end, this line is emphatic. The speaker feels strongly about what he's saying here.
Yet knowing how way leads on to way, I doubted if I should ever come back.
The speaker realizes that his hopes to come back and try the other path may be foolish.
He knows how "way leads on to way" – how one road can lead to another, and then another, until you end up very far from where you started. Because of this, he doesn't think he'll ever be able to come back and take that other path, as much as he wishes he could.
Here we return to the metaphorical meaning of this poem. In any life decision, we can hedge our bets by thinking we can always come back, try a different option later. But sometimes our decisions take us to other decisions, and yet still others, and it's impossible for us to retrace our steps and arrive back at that original decision.
It's like deciding which college to go to – "I can always transfer" a high school senior might think. But then, once the decision is made and freshman year has passed, the reality hits that switching schools is a lot more complicated than it seems, and it's hard to start completely over somewhere else.