Younger people in the smart phone age are used to people replying to them instantly or within 5 minutes.. They think you are shunning them if you do not reply quickly enough..
Of course if you are like me and access SW via a computer.. You will only see messages once you log into SW which for me might only be twice a day.. So a message to me can sit unanswered for 10 to 12 hours or more and if i miss a day because of work or obligations then it might be sitting in my message box for a few days unanswered..
But if they live in smart phone world i guess the idea of people not being connected 24 hours a day 7 days a week is so foreign and frightening to them that the thought of it freaks them out..