I think romantic love evolved to enable you to focus your mating energy on just one individual at a time, thereby conserving mating time and energy.
Romantic love allows you to focus mating energy. Attachment sustains that relationship as long as necessary to raise your baby.
It certainly would have been adaptive for ancestral man to have a chubby wife during stressful times of famine. Not only would she have had more calories to burn, and thus more energy and endurance, but since fat stores estrogen, she would have remained fertile for longer.
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