Focus on What You Have Instead of What You Don't

The grass is always greener on the other side.
남의 떡이 더 커 보인다.

Reference:
[1] http://quotesgram.com/img/grass-greener-on-the-other-side-quotes/14096277/
[2] http://m.blog.naver.com/daegoni86/70136325397