From what I'm seeing/ feeling, it's not necessarily a sensitivity to one thing or another, it's almost like an oversensitivity to life. I think I related a bit more as a child and toughened up a lot as I got older. (You need to.)
Yesterday, for instance, I was watching this older couple at work. The woman, mother of 10, has severe dementia. Very sweet tiny little old lady sitting in a geri chair. She had a bib on for some reason. The husband is still with it and came to visit her. He was wearing a little tan hat, kind of like a beret, but i'm not sure of the correct term. His facial expression was lost, and she looks like an elderly child. He was holding her face and looking directly into her eyes, bent over her like he was taking care of a baby, straightening her bib. I can't really describe what I saw in every single detail, but in one glance my heart immediately dropped into my stomach and tears came out of my eyes. It was really embarrassing. An aide was watching me, goes, "Are you crying?" (She knew what I was looking at.) It was so embarrassing that I had to go in the bathroom for a few minutes. (One of many examples.)
To me, that's a part of "highly sensitive" or "empathic." I feel like I just see too much sometimes. I'm happier when it's turned down or turned off.
It's really hard to describe, actually, to the point where I feel like I'm hardly describing it at all or making it sound fake. It's a relief to know that there's an actual word for it. (And that word ties a whole lot of crap together that otherwise wouldn't have been tied together.)
I'm reading more about the energies and qualities that can come with being very sensitive. I think it's something that manifests itself in different ways and the manifestations seem to be based on other personality characteristics combined with life experience.