Simply put? Because he had lost, and he didn't like the odds he faced otherwise. Even through early 1945 Hitler had refrained from admitting to anyone that the war was lost, and it was only in the final months that he was willing to accept the fact. With the Soviets nearing Berlin, the writing was on the wall and he finally had to consider the end. In his Political Testament dictated the evening prior to his suicide, he made clear his choice, and made brief attempts to defend it as well, noting both that he "[decided] of my own free will to choose death at the moment when I believe the position of the Führer and Chancellor itself can no longer be held" and that "I myself, as founder and creator of this movement, have preferred death to cowardly abdication or even capitulation." Now, while he might be trying to paint this as a noble choice, it was the fear of the alternatives that were just as much in his mind. I've written previously about Allied plans to deal with Hitler if he were captured, and this no doubt also helped to solidify his resolve. The prospect of capture by the Soviets, possible display to the world, and being tried in what he no doubt assumed would be a quick show trial followed by execution, none of that was exactly what he looked forward to. Even upon deciding to commit suicide, he wanted to ensure that his body wouldn't be turned into a war trophy, which was much of the reason for the cremation of his corpse following death.