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...
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 self, the latter was now much smaller than the former, just like a muscle that has been exercised will be much larger than a muscle which has not. It follows then that Mr. Hyde, like a muscle, grew more and more powerful as he was exercised more and more by Dr. Jekyll's addiction, until the point where he became too powerful for Dr. Jekyll to be able to suppress.
The second reason accounting for Dr. Jekyll's loss of control over Mr. Hyde is simply that Dr. Jekyll's first potion was impure, meaning that it contained a trace of something that he was not aware of, and without which the potion lost its potency. Dr. Jekyll acknowledges this, again in chapter 10, when he says, "I am now persuaded that my first supply was impure, and that it was that unknown impurity which lent efficacy to the draught." This is why, towards the end of the story, Dr. Jekyll, trapped in the physical form of Mr. Hyde, repeatedly sent Poole to the chemists to purchase different chemicals, hoping to discover the identity of the original impurity. By this point in the story Mr. Hyde had grown too strong for Dr. Jekyll to suppress without the aid of the potion, and so when Dr. Jekyll ran out of the potion (or, more specifically, the salt from which the potion was made) and could no longer recreate it, he lost all hope of ever wrestling control back from Mr. Hyde, so that his only remaining option was to commit suicide, and thus kill Mr. Hyde along with himself.