The Peter Paul and Mary website states that it is "a wound that never filled", which makes the most sense to me.

I think the interpretations by Mark (she's locked up within herself held captive by her fears), Craig (she's so hollowed out by lonliness that she can't weep for herself so we must weep for her, Leeneia (a deep clinical depression), Joe Offer (abuse by her father), John Hary (She doesn't know love because she can't give it) are all astute and attest to the power of this great song.

It does seem to me to be an excellent portrayal of a depressed woman. And she is haunted by some kind of recent loss and/or childhood trauma. My guess is that most of her acquaintances don't even know she's depressed--they're fooled by her "porcelain smile and wetless tears".

It also typifies the pressure everybody feels to focus on surface and the difficulty of (and sometimes the resistance to) knowing other people in a really deep way. It's probably even more relevant in today's narcissistic society where the focus is so much more on "image" than reality.