grass is always greener

The grass is (always) greener (on the other side of the fence) — Хорошо там, где нас нет

This idiom means that what other people have or do looks preferable to our life.

The complete phrase is The grass is always greener on the other side of the fence'.

см. тж.
[запретный плод сладок]
[those were the days]
the good old days