Most of our desires are accompanied by a feeling of pleasure, for the recollection of a past or the hope of a future pleasure creates a certain pleasurable enjoyment; thus, those suffering from fever and tormented by thirst enjoy the remembrance of having drunk and the hope that they will drink again. The lovesick always take pleasure in talking, writing, or composing verses about the beloved; for it seems to them that in all this recollection makes the object of their affection perceptible. Love always begins in this manner, when men are happy not only in the presence of the beloved, but also in his absence when they recall him to mind. This is why, even when his absence is painful, there is a certain amount of pleasure even in mourning and lamentation; for the pain is due to his absence, but there is pleasure in remembering and, as it were, seeing him and recalling his actions and personality. Wherefore it was rightly said by the poet: "Thus he spake, and excited in all a desire of weeping."
(Aristotle, Rhetoric; translation taken from www.perseus.org. The verse cited at the end occurs once in The Iliad with reference to the mourning for Patroclus, and once in The Odyssey with reference to lamentation due to the absence of Odysseus.)