Two paramount aspects of Dr. Jekyll's life led to his profound expression of life lived without restraints as Mr. Hyde in The Strange Case of Dr. Jekyll and Mr. Hyde. The first has to do with his character traits and the second has to do with his research. In "Henry Jekyll's Full Statement of the Case," Robert Louis Stevenson provides the background information necessary for answering this question.
Through Stevenson's pen, Jekyll explains that he has always felt strongly and nurtured within himself the two constituent parts that are said to make up all of humanity: the parts of the good and the evil. Jekyll also tells that his scientific research had turned toward the mystical and the transcendental. Mystical science means research with the intent to unlock the spiritual truths of the material things of the universe, such as asserting a spiritual nature underpinning the material world.
Transcendental science is closely related but encompasses the transcendence of matter, such as getting matter to change form, as Jekyll succeeds in doing. As a result, Jekyll has a sense of self-congratulation that gloats over doing charitable deeds in his good manifestation that compensate for the crimes and outrages committed in his evil manifestation. He makes this clear as he explains the thoughts he had while seated on the bench in Regent's Park on the January day of his fateful decline.
To further explain, Jekyll enjoys being Hyde because he found a way to separate his halves and allow his evil nature to fully satisfy itself. He also enjoys being Hyde because as a scientist he glories in his great accomplishment and tells himself that he wants to track how this "experiment" (as he calls it) with life progresses and yields satisfaction.