Euler is one of the few mathematicians who provide an exception to this rule. To quote Polya (Mathematics and Plausible Reasoning):

Euler seems to me almost unique in one respect: he takes pains to present the relevant inductive evidence carefully, in detail, in good order. His presentation is "the candid exposition of the ideas that led him to those discoveries... Natural enough, he tries to impress his readers, but, as a really good author, he tries to impress his readers only by such things as have genuinely impressed himself.

(the quoted passed in the text is apparently from Condorcet, although I don't know the initial source)

Polya is, of course, one of the few other mathematicians who break this mould. Explicitly writing books about the process of discovery.