We have all heard the saying, "I always want what I can't have."
I believe that human nature has increased our chances of survival. We tend to accomplish things, and we want more and more, sometimes things or people that are not likely for us to have.
In other instances, we are continually chasing happiness. Sometimes we see that other people have something that we don't and we believe that is the solution to our unhappiness.
Or it could be that subconsciously we know that item or person is out of our reach and it makes us want it even more because we know we can't have it.
The human mind can be weird at times. Sometimes our wants, do not need an explanation and sometimes they do.
In psychology, this thought process is referred to as the Hedonic Treadmill. It allows us to keep wanting to achieve throughout our lives.