It occurred to me that I don't agree with this notion.
Simchot are times when, by definition, the heart is light. Things are good. The clouds have parted. The angels are singing. So it's very easy to meet at simchot; very comfortable. Unhappy events are not when we are at our best. Crying does not do much for one's appearance.
On the other hand, somehow it's often when there is an unhappy event, that we become bound together. Relationships become solidified. The slag is removed and people's true essences (good or bad) shine through.
So when someone says, "May we meet only at simchot", they could be saying, "I really don't want a relationship with you with any depth. Let's just keep things at the nice and comfy surface, okay?"
On the other, other hand, pain sucks. It's necessary, and part of life, but no one likes it very much. (Except those people who are just unhappy unless they're unhappy - and we all know people like that!) So we hardly want to be saying, "May our next meeting be in sorrow." It's just too intense.
So what should we say? Or at least, what should we mean, regardless of what we say?
I think both poles are necessary for a good relationship, and everything in between as well. So what we really should mean is something along the lines of "May we have other occasions in which to share each others' company," or "May we see each other again soon," or even just, "See ya'!"
The thing is, we also always want to end things on a "better" note. We're just wired that way. We're uncomfortable with bad stuff, which makes sense since it's necessarily uncomfortable.
Chag kasher v'sameach to all...