Explanation for why displeasure would be associated with meaningfulness, even though in fact meaning comes from pleasure:
Meaningful experiences involve great pleasure. They also may come with small pains. Part of how you quantify your great pleasure is the size of the small pain that it superceded.
Pain does not cause meaning. It is a test for the magnitude of the pleasure. But only pleasure is a causal factor for meaning.
My hypothesis is that it's pleasure. Or more specifically, whatever moral argument most effectively hijacks an individual person's psychological reward system.
This just kicks the can down the road on you defining pleasure, all of my points still apply
If instead, pleasure is a particular phenomological feeling similar to feeling happy or content, I think that many of us actually WANT the meaning that comes from living our values, and it also happens to give us pleasure.
That is, I think it's possible to say that pleasure kicks in around values that we really want, rather than vice versa.