I've mused about the psychological underpinnings of witnessing before. But recently I had a personal experience that made me think about it in a slightly different light.
We have a Nursing Nina toy - a stuffed cat with three kittens, with magnets that let the kittens "latch on" to the mother cat's nipples. It's totally adorable and funny. I had been meaning to share it with my friend who was coming over for a visit, since I thought she would get a kick out of it and, frankly, I wanted to show it off.
The morning of the visit, she called to apologize for being late. One of their dogs had attacked one of their kittens that morning, and she had rushed the broken and bleeding little thing to the vet, only to have it expire in the car. After we got off the phone, and my mind had turned to humdrum house stuff as I tidied up, I came upon Nursing Nina and my first thought was of how cool it is. Then I checked myself and realized of course I'd better put it away to avoid rubbing salt in my friend's wounds.
I think hard-sell witnessers are in part like kids wanting to show of their cool toys, without any regard to the interests or feelings of the people they're showing them to. Even if their intentions are good, there's an underlying self-centeredness that's really off-putting.