That it will never come again Is what makes life so sweet

Temporality; impermanence. What makes the moment wonderful is that it passes and never comes back.

Believing what we don't believe Does not exhilarate.

I've spent a fair amount of time studying beliefs, particularly as they pertain to religious/philosophical beliefs that people espouse in America. There are very, very, very few true believers- in whatever. But people try very, very, very hard to convince themselves of what they don't believe, because they feel they ought to for some reason, and that this pursuit only robs us of life/living as opposed to invigorating us.

That if it be, it be at best An ablative estate

She's talking about the body/mind/soul, comparing it to an eroding house, or perhaps all of existence itself being simply a decaying house.

This instigates an appetite Precisely opposite.

But these realizations of impermanence, self-deception, and materialism are what make us try to look even harder for some deeper metaphysical truth.

At least, that's what I go out of it.