And I'm not just talking about unresolved childhood issues. I'm also talking about adult issues.

They could be unhappy about anything. Seeing Dad happy with SP might highlight to them how they do not have a romantic partner. How they aren't getting that happiness in their lives. Maybe that's why it could be tough for some adult SKs to do stuff like have dinner with Dad and SM, and see Dad and SM sitting next to each other, happy. It might very well remind SK of how UNhappy they are with life. They could see the dad and/or SM flourishing in any aspect of life and feel unhappy because they are not.

I'm dealing with some friends who feel this way about their parents and SPs. They do not hate the SPs but feel left behind, unfulfilled and unhappy in their lives and seeing Dad and SM so happy really emphasizes that to them.

What advice can I give to these people? What advice would you give to adult SKs in this predicament, and how do you think adult SKs should handle themselves in these situations? What do you think they could/should do to imrpove things for themselves?