"The grass is always greener on the other side." is a phrase people use to mean that "somebody else's situation always appears better than your own when you're looking from the outside." The truth is that everyone has their own select problems. I find a good nugget of wisdom is that "The grass is greener where you water it." which means essentially the same thing as "You reap what you sow." You can't get anything of value out of life if you don't put in time trying to cultivate things of value. Hope this clears up the meaning of the phrase :)

This is based on the saying 'The grass is always greener on the other side of the fence', which means that people are never happy with what they have in life - we always think that our neighbour has a nicer garden than ours. It's human nature to imagine that other people are luckier than you, to be jealous of what other people have, or to believe that we would be happier if our situation were different.