itchy feet: if your feet are itchy you always have to desire to go travelling, to leave your home and set our for new pastures.

The grass is always greener on the other side: Used to describe a state of mind whereby the things one has are never as good as the things one doesn't have. Just like the unsatisfied cow who looks over the fence and sees the green field opposite where the grass looks much better. For instance, if your old girlfriend is always better than your present one or your old job seems better than your new one, then for you the grass is always greener on the other side of the fence.

A: "I don't know. I just feel like I've got to get away. This place is too small for me. I feel like there's so much of the world I haven't seen."
B: "But you've been travelling all your life. Aren't you tired of it yet?"
A: " No not at all. I've always had itchy feet. And once you give in to the urge, the desire to move just gets worse and worse. I can't stay in one place for more than two years before I'm off again."
B: "Aren't you ever going to settle?"
A: "Who knows. Perhaps if I find a place I really like. But you know, the grass is always greener on the other side. I'll think one place is wonderful and want to stay there, but before long I'm thinking about somewhere else and the place I'm in just seems dull. So I move."