We toil because we are literally made to do so. Not to toil would be like being a cheetah that refuses to run.
If we try in every moment to cultivate gladness of heart -- completely within our power -- our toil will have purpose and our burdens will lighten. It could hardly be otherwise.
If, on the other hand, we try to manufacture gladness of heart through acquisition and accumulation of possessions -- be they animate or otherwise -- we are trying to make an exquisite cake by throwing the entire kitchen into the pan.
We toil because toil, ironically, is the most fertile ground for gladness, the very seedbed of joy.