According to Founding Brothers: The Revolutionary Generation, why was the friendship between Jefferson and Adams so central to the development of...

The friendship between Adams and Jefferson was one that
transcended political differences.  Essentially, Ellis argues that friendship and a
sense of mutual respect was critical to the development of the new
nation:

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...character mattered because the fate of the
American experiment with Republican government still required virtuous leaders to
survive.

Due to the fragile
state of the nation and the experiment, in general, Ellis argues that the friendship and
respect that the framers held for one another was the glue that allowed the nation to
"congeal."  Ellis demonstrates this in his detailing of the friendship between Jefferson
and Adams.  Theirs was a predicament where political rivalries were intensely present. 
Jefferson and the Republicans were vociferous critics of the Adams presidency.  Yet,
after both left office, their friendship resumed through correspondence, something that
represents how both essentially held respect for one another outside of politics.  This
is the type of friendship that showed how the fragile nation was held together because
of mutual respect for one another.