"I think you'd have to be very insecure about your own situation to think anything more than "ah that's nice" to pictures of happy families on Facebook."
Insecure or maybe sad, lonely, living in poverty, recently bereaved/separated or just able to empathise with people who are going through any of that.
And it's not happy families is it? Nobody objects to a cute photo of your kids opening a gift, your family round the dinner table or even the now-obligatory 'matching pjs' photo. It's the 'look what I've spent' posts that are so irritating.
"Yes, I would think that bitching about how other people celebrate Christmas is absolutely a sign that you aren't happy with your own lot."
Of course some people aren't happy with their own lot. Of course some people are bloody suffering. What amazes me is that you, and people like you, decide to shove your massive pile of gifts down their throats anyway.
So anyone who has less than you feels a bit crap, and anyone who has more is probably laughing at your giant pile of Primark tack. It makes no sense. It is for no purpose or point other than to show the world what you've spent, but the world doesn't care.