grass is always greener on the other side, the

Cf. [Ovid Ars Amatoria i. 349] fertilior seges est alienis semper in agris, the harvest is always more fruitful in another man's fields.

The grass is greener.
[1959 H. & M. Williams in J. C. Trewin Plays of Year XIX. 13 (title)]

‘The grass always looks greener on the other side of the fence,’ said another informant, explaining that while stores who do practise the system are uneasy about it‥those outside constantly wonder whether results might not justify it.
[1965 Which? Mar. 91]

They push their heads through fences and get stuck (the grass on the other side really is greener: sheep invented the axiom).
[2001 Spectator 15/22 Dec. 26]

Related to: content and discontent

Bibliography of major proverb collections and works cited from modern editions is available here.

A different situation always seems better than one's own. For example, Bob always thinks the grass is greener elsewhere, which accounts for his constant job changes. This expression, an ancient proverb cited by Erasmus in the 15th century, is so well known that it is often shortened.