FDR was throwing in with Uncle Joe to rule the post war world because he thought he was somebody he could deal with.
International relations are all about power. The USSR was going to have it, the British Empire was ready for a “going out of business sale.” The adults who play international politics as a serious game with high stakes don’t play for sentimental favorites.
The cold reality was that the United States and the USSR were going to be the two top powers in the world, so it makes sense to deal with each other. What didn’t make sense was trying to trust a guy like Stalin. To the extent Churchill was trying to warn FDR of that, FDR was foolish to not listen. Unfortunately, Chuchill also spent too much time trying to get FDR to preserve the British Empire, and that was a non-starter for FDR.
PS: I sometimes worry that the United States today is in the same relative power position as the British Empire at the close of World War 2, but maybe worse off. Barack Hussein 0bama is no Winston Churchill.