Why does Jekyll's lower nature come to dominate him?

In pursuing his scientific experiments and validating his work, Jekyll claims, "man is not truly one, but truly two." Thus, in Jekyll's view, every soul contains elements of both good and evil, but one is always dominant. In Jekyll's case, his good side is dominant, but he knows there is evil inside of him. However, as a respectable member of society and an honorable Victorian gentleman, Jekyll cannot fulfill his evil desires. Thus, he works to develop a way to separate the two parts of his soul and free his evil characteristics. Victorian society was full of strict manners, emotional control, and repressed primal desires. Jekyll longed to indulge himself in the vices that he was expected to control.