You wonder why tiny Finland dared to challenge the gigantic Soviet Union? The reason was plain and simple, to avenge its lost territories & attempted to get the lands back. Finland couldn't do it with its own power so it put its hopes on the Germans (I believe that the Finns only fought against the Russians during the war). Romania participated in the invasion of the Soviet Unions with the same reason.
Let me summarize the events before the invasion of the Soviet Union.
08.24.1939, The Molotov–Ribbentrop Pact was signed.
09.01.1939, Nazi Germany invaded Poland from the west.
09.17.1939, the Soviet Union attacked Poland from the east. After defeating Poland, these two nations divided Poland.
09.28.1939, the Soviet Union annexed three Baltic States.
11.30.1939, the Soviet Union invaded Finland to start Winter War.
03.12.1940, both sides signed Moscow Peace Treaty to end Winter War but the Finns lost 10% of its territory.
06.28.1940, the Soviet Union annexed Romanian's Bessarabia, Northern Bukovina and the Hertza region.
By early 1941, the German and Soviet empires shared a common border running through what is now Lithuania and Poland.
My point is that 苏联本身也是侵略者 before the invasion. Stalin wanted to
share Europe or the world with Hitler but Hitler wanted it all.
10.1940, Stalin sent Molotov to Berlin to negotiate the terms for the Soviet Union to join 轴心国 (I am not 100% sure about this)
Ribbentrop welcoming Molotov in Berlin, November 1940