True love does not quarrel for slight reasons, such mistakes as mutual acquaintances can explain away, but, alas, however slight the apparent cause, only for adequate and fatal and everlasting reasons, which can never be set aside.
 Henry David Thoreau, A Week on the Concord and Merrimack Rivers (1849). copy citation

Context

“They must apologize like dew and frost, which are off again with the sun, and which all men know in their hearts to be beneficent. The necessity itself for explanation,—what explanation will atone for that? True love does not quarrel for slight reasons, such mistakes as mutual acquaintances can explain away, but, alas, however slight the apparent cause, only for adequate and fatal and everlasting reasons, which can never be set aside. Its quarrel, if there is any, is ever recurring, notwithstanding the beams of affection which invariably come to gild its tears; as the rainbow, however beautiful and unerring a sign, does not promise fair weather forever, but only for a season.” source