The grass is greener on the other side of the fence.
Meaning: People are never satisfied with their own situation; they always think others have it better.
Example: X: When I see him post all those travel pictures on Instagram, I feel he has the perfect life. Y: It’s usually not like that in real life. I’m sure he too has his share of problems. I see your thought as grass being greener on the other side of the fence.