The Grass Is Always Greener on the Other Side of the Fence
(another place or situation always appears to be better than your own)
Jim and Alex had been friends since childhood, but as adults their lives went in different directions. Jim achieved great success in business, while Alex worked as a clerk in a department store. Although Alex was basically happy with his job, he would often think that the grass is always greener on the other side of the fence when he considered Jim's financial success. It appeared to Alex that his friend was much better off and in a more advantageous position than he was. What Alex did not know was that Jim was not happy with his job and that his health was suffering because of the constant stress.
be better off = もう一方よりもいい生活をしている
（cf. be well off = 裕福な暮らしをしている）