People who are being overly emotional, in sadness, anger, jealousy or other ‘negative’ emotions, bring down the mood of the situation, so social convention naturally tends to discourage such behaviour via direct or implied criticism.
It is so pervasive and effective, that nobody need actually make the criticism. We all know nobody wants to see us crying or shouting.
In contrast, emotional outbursts of positivity are generally not considered detrimental to most social situations, unless taken to the extreme. And, even if one is annoyed by them, dissatisfaction is generally not voiced due to the very same pressures, described above, against manifesting negativity publicly.
There is no ‘need’ to comment on people being emotional in a socially acceptable way, so it is rare to hear, and such a comment would imply that the person in question does not normally behave in that way.
I think it works pretty well.