In general, Stalin believed that a confrontation with nazi Germany was inevitable and that he needed time to prepare the Soviet Union for this war.

Stalin wanted a buffer zone, and did negotiate in good faith with the Allies regarding an alliance, but they would not budge on a buffer zone, and the level of diplomats sent were on a too low level for a serious discussion on issues Stalin actually wanted concessions on. So when the Germans offered what would become the Molotov-Ribbentrop pact, Stalin grabbed that potential with both hands. He would get his buffer zone and time to reform and re-arm the Red Army.

One needs to remember that up until 1941, a large part and in some cases a majority of the Red Army's heavier equipment were upgrader or non-upgraded relics of the Imperial Russian Army, especially when it came to artillery and AT guns (the Soviets used a lot of former anti-torpedo boat guns of 37mm calibre as AT guns).

Stalin, probably like most of the contemporaries, expected Germany to be bogged down invading western Europe, and his greatest fear, the "imperialist" (read democratic) and facist countries uniting against him and his Societ Union would be averted. A dragged out ww1 style conflict where the Allies and Germany exhausted each other, while the Soviet Union re-armed and re-builkt, safe behind its buffer zone suited Stalin just fine.

However, that did not happen- France fell and the British retreated from the continent and suddenly Germany was the hegemon of Europe.

However, Stalin wanted to believe that the Germans would not attack the Soviet Union before they had conquered Britain, and he had decent reasons to believe so. The Germans were since ww1 wary of a two-front war. Germany was more or less dependent on raw materials shipments from the Soviet Union to maintain their production and the German actions elsewhere seemed to support this. The Italians and then the Germans fought in East Africa and Libya and through Syria tried to support the Iraqi revolt against Britain in May 1941. The Germans also seemed to focus on raiding the British trade lanes, both with submarines and heavier ships as the Bismarck sortied and was sunk.

The Germans also invaded Yugoslavia to help the Italians secure Greece before British aid could arrive.

It seems like the many warnings Stalin did recieve - from spies, from border troops and other souces, including Sweden which had broken the German geheimschreiber encryption machine - were ignored , likely because of his paranoia, as attempts by British intelligence to sow split between the Soviet Union and Germany and potentially cause conflict between them.

To Stalin, it probably seemed likely that Britain would attempt this - they had supported, some even say initiated the coup in Yugoslavia that caused the country to reject German advances and Germany to invade it and it looked like the Germans were pursuing a periphal strategy against the British.

So, the bottom line is that Stalin did not want to believe the Germans would attack in Summer 1941, and with his paranoia and some of the facts on the ground, he found plenty of reasons to not believe that. It was not as much that he trusted Hitler, he fully expected him to break the pact sooner or later, but rather that he believed Hitler would not launch a second front before dealing with Britain, and any indication on the difference was British intelligence psy-ops.