One extremely important purpose of emotions from an evolutionary perspective is to help us decide what to remember and what to forget. The cavewoman who could remember which cave had the gentle guy who gave her food is more likely to be our foremother than the cave woman who confused it with the cave that held the killer bear. The emotion of love (or something resembling it) and the emotion of fear would help secure her memories.
Molecules Of Emotion: Why You Feel The Way You Feel (ed. Simon and Schuster, 2012) - ISBN: 9781471109706