Answer who can!

I have an old friend. Ten years ago, he suddenly became a vegetarian for various reasons. After a while, after such a change, I suddenly began to notice that his beautiful sense of humour had declined.
Hence the question: are these things (vegetarianism and YU) correlated, or is it something else? The chemistry of the body is changing.
ZY: on thematic sites such questions are better not to ask, checked.