In the books I've read I've never seen any opinion that Stalin's belief was in any way the result of a rational examination of the available evidence. From Mein Kampf, Stalin knew that Hitler intended to attack the Soviet Union. He also knew that Hitler had abrogated peace treaties in the past, indeed had helped Hitler invade Poland. Stalin, and the Soviet High command, ignored numerous warnings including :
Stalin appears to have acknowledged that war with Germany was likely, but became fixated on two beliefs. Firstly that the Germans would not attack only two years after signing a non-aggression pact with the USSR. Secondly that the British wanted to provoke the USSR into declaring war on Germany.
This second belief basically allowed him to ignore any evidence that contradicted the first belief. It's possible that the flight of Rudolph Hess to Britain in early May 1941 reinforced the belief that Churchill was actually colluding with Hitler. Even then that doesn't explain the lack of warning to the Soviet border forces.
Sources:
The Second World War; John Keegan. The Second World War; Martin Gilbert. Barbarossa: The Russian-German Conflict, 1941-45; Alan Clark. When Titans Clashed: How the Red Army Stopped Hitler (Modern War Studies); David M. Glantz. The Rise and Fall of the Third Reich; William Schirer.
Your question is heavily dependent on what time frame we are talking about. Years before the conflict began? Months? Weeks?
In the long term, Stalin certainly believed that a war with Germany isn't just likely, it was almost inevitable, given the ideological split between the two nations. Stalin has done a lot to prepare for this war - everything from militarization of the Soviet industry, reforms in the Red Army, attempts to move the border westward, and so on. For a time, Stalin tried to make an alliance with Western powers against Germany, but was unsuccessful. The non-aggression pact with Germany was a response to that. Stalin wanted to make sure that if the war was inevitable, the USSR wouldn't be the first in the fight. He wanted to buy time for when the Red Army would be ready for the conflict.
In the medium term, as Hitler was conquering France, Scandinavia, the Balkans, etc, Stalin thought the war was unlikely. There were two main reasons for that. One, there weren't any significant diplomatic disputes between Germany and the Soviet Union. Yes ideological differences exist, but they were not enough to start a war. Usually there is a period of tension. For example, WWI began with the shooting of Archduke Ferdinand, and the Austro-Hungarian ultimatum to Serbia. But wars don't just happen in a vacuum. Secondly, the was still a war going on. Great Britain was still in the fight. Stalin did not believe that Hitler would attack the USSR before finishing off Britain, as that would be a very reckless thing to do. In fact, some intelligence that he received did suggest that Hitler was planning to invade Britain.
In the very short term - we are talking a few days to a couple of weeks before the German attack on the Soviet Union - Stalin did begin to appreciate the likelihood of an imminent invasion. Some preliminary steps were taken to prepare the Red Army for war. However Stalin simply lacked good options at this point. There was little he could do short of ordering a full scale mobilization, which would have made him look like the aggressor. This difficult situation resulted from the fact that Wehrmacht was already mobilized, and the Red Army was not. And the RA needed a lead time of over a month to mobilize. An order to mobilize would have been a very clear provocation (as was the case in WWI) and a signal to Germany to "attack now". In any case, at that point it was too late to do much about the situation.
Regarding intelligence, things were not as clear as they are sometimes made to be. Stalin received a lot of reports, some suggesting that war was imminent, others showing the opposite. Soviet intelligence analytics were quite poor at the time. Knowing which sources were reliable and trustworthy, and being able to connect the dots was difficult in a sea of often contradictory information. For the most part, Stalin tended to rely on reports of German troop concentrations which, until very late in the period we are talking about, were ambiguous. For a while it appeared that Germany was simply putting in place an infantry screen to guard against a possible Soviet invasion. Germany armor units were still in the Balkans after the Greek and Yugoslavian campaigns. The infantry screen appeared to indicate a defensive posture and that Germany might attempt an invasion of Britain.
Sources: Исаев А. В. От Дубно до Ростова
Soviet Storm, documentary series.
I don't think that much of what people do can be considered 'reasonable'. Perhaps 'understandable' would be a better term. Stalin was absolutely desperate to avoid war, and had managed to convince himself that Hitler wouldn't invade, even while preparing his army for an offensive war at the same time. As the evidence of war loomed, he obsessed over the history of the German-French war of 1870. He put himself into the mind of Hitler, and being a 'Bismarkian Great Power Statesman' type himself, he convinced himself that Hitler was as well. He and Zhdanov quoted to each other Bismarck's famous dictum that Germany should never fight a war on two fronts, and took this as the gospel Hitler would live by, ignoring the fact that Hitler was an idealogue, not a player of the Great Game.
But Stalin, like those who prospered under him, had the power to believe different things to be absolute truth at the same time. While being certain that Hitler wouldn't invade while England was undefeated, he was also aware of the looming invasion as a reality, and hoping that Molotov would delay it. On 5th May he announced at dinner that "if VM Molotov can delay the start of war by 2-3 months, this will be our good fortune". This was partly an attempt to bolster the officers present for ideological reasons, but the fact that Stalin could even say this in public meant that he wasn't as completely blind to reality as many have portrayed him.
Stalin knew that Russia wasn't ready for war in 1941, he hoped to delay it until 1942 at least, saying that Russia wouldn't be truly ready to meet the Germans on an equal footing until 1943. Because of this, it was absolutely necessary for Russia that Hitler not invade in 1941, so because of the absolute necessity of this, Stalin acted as though merely by willing it to be true, it could become so. He knew the storm clouds were closing in, but fighting against it, both with his indomitable will, and any action he could take, however slight. He was terrified of doing anything that might provoke Hitler, refusing to let famous literary author Ehrenburg mention the word 'fascist' in their book, and considering cancelling the May Day parade in case the Germans found it provocative. This shows that Stalin was on some level aware of the reality of the situation, and that he hoped that by such insignificant gestures he might avoid invasion for as long as possible.
Finally, as May drew on, he began to mobilise the army and prepare it, but slowly and cautiously, fearing that the slightest wrong step might antagonise the Germans. On 12th of May he allowed his generals to call up 500,000 reserves. But after German reconnaissance flights, on the 24th he refused to allow any further measures. Struck with paralysis, perhaps he feared that anything he did might precipitate the catastrophe he was so desperate to postpone.
Source: Stalin: The Court of the Red Tsar, by Simon Sebag Montefiore
As a follow-up question, I watched a video in my history class that claims that one of the main reasons Hitler decided to invade Poland was so that he could have a shared border with the USSR. Can anyone confirm or deny this?