Got an Answer?
Because they feel the need to compete with others who have bragged about their fabulous life? Maybe they're not bragging at all but complaining because they feel they've been overcharged? I complain when I feel like I've paid more than something is worth. Like when you end up paying $8.50 for a coffee (I'm looking at you Sydney!) and it's burnt and bitter.
We all compare ourselves to others, that's just human nature. We all see what everyone else does and feel we have to one up them, some are obsessed with it. Social media really has made the world a worse place.