I understand the frustration at comments that imply that people are thinking about how others should live their lives. Being surprised that someone hasn't moved house, or taken in an aged parent, or had a second baby, or remarried, or whatever suggests that more thought than is necessary has gone into someone else's business. It's the same with 'I always thought that' Fred was gay, or Sally didn't want children, or they must have won the lottery etc. Why would someone 'always think' about the lives of their neighbours unless they were very bored?
Maybe a raised eyebrow, and an 'So, is there anything going on in your life, then?' comment would get the point across?