Oh, what an intriguing question! I think that the symbiotic relationship between a breast-feeding baby and a mother (human or otherwise) is very endearing. The profound emotions that the mother experiences while nurturing her child are only replicated by the love and utter trust that the baby feels for her. But when there are two adult humans still "joined at the hip," so to speak, still dependent on each other to live, it's not quite so endearing and is labeled "enmeshed" or smothering. The trick is to have an adult relationship that is healthily interdependent without either party thrown into high anxiety and panic attack if the other person is not available to meet all their needs.