Our fearless researcher Hannah dug up a bit of the literature about reciprocity's relationship with interpersonal attraction.
Reciprocity – simply liking someone who likes you - was explored in a study by Aronson and Linder (1965) in which participants "accidentally overheard" another subject (primed by the researcher) express a liking or disliking of them. The subject was then asked to fill out a questionnaire expressing their like or dislike of the subject.
Perhaps unsurprisingly, the questionnaire mirrored the overheard comments - a confederate who had made positive comments about the subject was liked; one who made bad comments was disliked. However, a third group heard the confederate start by making bad comments and end by making good comments. These subjects, who thought they had "won him over," gave the confederate the highest ratings of all.