The first sentence refers to Daisy: the sole and intense purpose of Gatsby's existence. She represents that thing "beyond the stars" that Gatsby aspires to. His entire adult life has been devoted to becoming the kind of man he thought would be worthy for Daisy to marry. Daisy's family was wealthy and socially respectable; Gatsby came from nothing. Their youthful love affair ended in tragedy because Daisy felt she couldn't marry someone with no money or social standing. In becoming wealthy and socially viable, Gatsby also became decadent. The "purposeless splendor" of his lifestyle is revealed to be far deeper and more significant, as Nick realizes Gatsby's singular purpose is to win back Daisy. In Nick's eyes, this makes Gatsby even more impressive and admirable, because Gatsby represents chivalry and romance in a decadent modern age.
The last paragraph explains that too many false alarms can be dangerous to one's health, which is important to explain since the rest of the passage seems to promote stress as a way to survive or thrive in moments of crisis.
