I think that some of the important quotes have to relate to emotional disposition and how individuals view consciousness through their emotions. Macon is the subject of this throughout Tyler's book, so the quotes that describe him are highly significant. Sarah might not be the best of characters for Macon, but her description of him is fairly accurate in terms of where he was and from where he must progress:
....there's something so muffled about the way you [Macon] experience things ... You're encased. You're like something in a capsule. You're a dried up kernel of a man that nothing real penetrates.
To a great extent, Sarah is right about how Macon perceives reality and how he views it after the death of his son. The idea of "nothing real penetrates" and Macon's status as a "dried up kernel" is reflective of the emotional desert that Macon has created for himself. It is a scenario that he finds convenient for himself, but one that will inevitably destroy his own sense of living and being in the world over time. It is also one that is highly difficult for other people to share life with Macon in such a condition, reason for Sarah's departure in the first place as he was unable to provide for her needs in the grieving for their lost child.
This is not to entirely say that the emotional failure that Macon demonstrated is something on his shoulders entirely. Tyler is really quite profound in suggesting that a person's inability to share emotional explorations of being in the world could be due to their relationship with another person. Sometimes, our moments where we are emotional cripples is because of the person with whom we are. The people we interact with help create an emotional abyss in which we are all too happy to enter. It is a very interesting dynamic that Macon brings out when he is describing his relationship with Sarah, and who he was in it:
[Macon was]locked inside the standoffish self he'd assumed when he and she first met. He was frozen there ... Somehow, his role had sunk all the way through to the heart.
I think it's significant that Tyler uses the concept of "the standoffish self." There are times when people meet other people in a particular frame of reference. In order to continue acceptance and the relationship, individuals continue to remain in this condition for sustenance. Over time, the persona they assumed at the start of the relationship becomes who they are in it. In this, Tyler does not place blame, and does not say that it is anyone's "fault," but rather it is one where people are simply not their best when they are with particular people. In think that both of these quotes speak to the complex emotional reality that relationships foster.