Naftush
Very simple. The single girls and guys often use the good shabbos as a way to casually flirt. A few years back I was in such a place where gut shabbos is exchnnged betwenn both genders and it was abundantly clear to me that some girls were flirting with their enthusiastic good shabbos and with their coy smiles. As for the guys they were only too happy to say good shabbos to attractive dressed up girls. I would say as a rule that singles should not be saying good shabbos to each other as it can be flirtatious and gives an opening to start a conversation which im sure happens often.
Now for married people I think theres less of a problem but any time the good Shabbos is not 100% lishma as just a greeting its a problem.