There is nothing wrong with hard work. Being industrious is a virtue that typically produces many benefits. Labor, as it is used here is something completely different. The Hebrew term used here implies sorrowful or wicked labor. The best way to understand this is to examine the behavior of the 5,000 people that Jesus fed in John chapter 6.
After this miracle, the people "labored" to find Jesus, even to the point of crossing a sea to another country! Note carefully what Jesus said to them,
"...you seek Me, not because you saw the signs, but because you ate of the loaves and were filled. Do not labor for the food which perishes, but for the food which endures to everlasting life..." -John 6:26-27
In other words, what you labor FOR determines if your work will ultimately produce fulfilling and lasting results. If your goal is to pursue wealth for the sake of comfort and the pleasures of life, sorrow is in your future. The question we need to ask ourselves is, what do we work for and why? There is a reason that there is something called a "rat race".