Why do people say life is not meant to be lived alone?

I think it's because most people are not built to do well alone. It doesn't seem to be just mental thing but physical too, since there were cases that showed that people with good social support heal faster than those who are alone. But I'd correct it that life is not meant to be lived without someone to lean on, not exactly alone, as it's definitely better to be alone that living with someone abusive.