In the beginning of The Strange Case of Dr. Jekyll and Mr. Hyde, Dr. Jekyll is in control of Mr. Hyde, and at the end of the novel Mr Hyde is in control...

There
are two main reasons as to why Dr. Jekyll lost control of Mr. Hyde. The first is that Dr. Jekyll
essentially became addicted to Mr. Hyde, in much the same way as a drug addict becomes addicted
to drugs. Being Mr. Hyde gave Dr. Jekyll such a thrill, and made him feel so liberated that he
couldn't resist taking the potion again and again. He says, in chapter 10, that the
transformation "braced and delighted (him) like wine." The more often he became Mr.
Hyde, the stronger Mr. Hyde became.

In chapter 10, Dr. Jekyll remarks that
when he first transformed, he was, as Mr. Hyde, "so much smaller, slighter and
younger" than he was as Dr. Jekyll, and he accounts for this by explaining that the part of
him which became Mr. Hyde, had, over the course of his life, "been much less
exercised." In other words, because Dr. Jekyll, as a Victorian gentleman, had had to
exercise his respectable public self much more often than he had had opportunity to exercise his
much less respectable private...