Why??

Very Simple: to save face and cover up the retreat and defeat from Moscow; and he achived it to a certaint extent.

He did it again with Kursk; he called it off to "help" the italians against the sicily invasion. Actually Germany was defeated.

pepe

"Tell me what were their names,
tell me what were their names?
Did you have a friend
on the good Ruben James?

Germany was doing a damned good job of pressing the issue on their own. Thirty-seven days before Pearl Harbor, the U-522 sank the destroyer Reuben James, killing 119 men of her crew. Fourteen days before that, U-568 put a torpedo into USS Kearny. The ship did not sink, but 11 men lost their lives. Neither of these incident equat to a Luisitania type sinking, but after the Greer Incident, the U.S. Navy received permission to fire at German submarines in self-defense. A few more torpedo attacks or the sinking of a U-boat or two, and war might have happened anyway.

On December 8, in his speech to Congress requesting a declaration of war on Japan, Roosevelt stated that since the attack the day before "a state of war has existed between the United States and the Japanese Empire." If Roosevelt believed that a war already existed with Japan, then why did he need Congress to declare war? There are obvious political and public reasons for doing so. But could Roosevelt have used the same logic against Germany? Considering the heightened tensions and the loss of American lives by German hands, Roosevelt might have simply stated that, for all practical purposes, a state of war existed between the U.S. and Germany. Roosevelt could then have escalated military operations against Germany without a formal declaration.

This would be, of course, a potentially dangerous political action to take. From a historical standpoint, in our era of debate over the war-making powers of the President, it is also Constitutionally problematic. For a politician like Roosevelt, however, that would certainly free his hands.

Do you think Roosevelt would have even asked for a declaration of war on Germany, if Hitler hadn't declared war first?

Having said that, there is quite a difference between Military Intelligence and Political estimates that should have been done by the German Foreign Ministry. The real question here should be what was Ribbentrop telling Hitler? As Ribbentrop is usually depicted as a Hitler lackey, I would think he was saying what he thought Hitler wanted to hear.
Which takes us back to the original question of What was Hitler thinking? I think you can find a good discussion of that at:

http://www.historynet.com/hitlers-dark-december-1941.htm

or in the book "The Wages of Destruction" which many consider to be the definitive book on World War II.