The grass is always greener on the other side. It is greener, at least, until you get there. We all want what we don't have. We want the nicer home until we are stuck with the larger mortgage. We want the better husband until we realize that he is a cheating fool and the previous guy was loyal to no end. Things always look better from the other side but they are very rarely better when you get there.
In the analogy of the grass being greener on the other side, we presume the grass to be greener on the other side because we as people have nothing more in life than to look forward to greener pastures. Assuming a pointless future makes our lives pointless, we as humans need something to look forward to so assume that what we don’t have is better.
Just like one should never say something might never happen, one also should never quantify any proposition is always. For example, whether the grass is greener depends on which side of the fence you are on. If you are on the green side of the fence, the grass on the other side is browner, not greener.