I had something pretty similar happen.
This dates back to when the earthquake happened here in New Zealand, it was only a few weeks after it happens IIRC so some major cleaning up was still happening, people were still pretty shaken up and nerves were still pretty frayed. I was working in a computer store as a sales person, pretty much the only one that was open in the city at the time. Business was booming for us (it was just unfortunate that it couldn't have been under better circumstances) and we were getting people from all walks of life coming in through the door.
Anyway, one day this guy walks in with his girlfriend. I let them walk around the store for a few moments then go ask if there is anything that I could help with, turns out he's looking for a fairly decent gaming computer. We go through the process of figuring out his budget, if there's something in particular that he wants, what he's actually going to use the computer for so I can best match his hardware requirements and eventually we get everything settled.
So I'm grabbing his details and getting payment for the computer and while I'm going through these motions, I'm making small talk and out comes some wise crack from me about him just getting this so he can just play some game that has just being released or is about to get released. He looks at me evenly and without any hesitation he says "Actually, no. I'm buying this computer to cheer myself up because both of my parents got killed in the CTV building."
My first thought was just like the OP but it was a very fleeting feeling, after that I felt pretty [censored] small and apologetic. He said not to worry about it, he'd had enough sympathy from everyone else around him which he said he found unbearable and he didn't need any more from me.
I could understand that and we both got on with it.