I am trying to track down a short story collection (Sci-Fi) that contained this story.
A couple have a crystal tree that reflects their emotions. They are very proud of the tree, even though they begin to not get along and even begin to loathe each other. They only show the tree what they want it to see and end up staying together longer because of the tree. So in effect the tree is only reflecting their behaviors not their true feelings. I think the tree thrived when it reflected them doing well, but when they did not do well the tree would not do well either.
The reader who asked me to help find this short story says he read it in the period from 2002 - 2005 but does not feel sure if it was written then or earlier. He thought it might have been in one of the "best of" sci-fi writing titles.