Why did the cold war start

Why? Well. Easy question, but not so easy to answer. In the final years of the war, it became clear, that the inevitable defeat of Germany will not be the end at all. The relations between the West and the East were cooling off since pre-war times and as the short-timed cooperation phase of the war was coming to an end the old rivality was showing up again. Different as the parties in the conflict were, they've chosen different ways to enter the new field of post-war policy.

The Western allies had made it clear, that they all fight the Nazis, but their next object is the protection of the world against the communistic Soviet Union. Feeling threatened by the spreading influence of the Soviet regime, Western world wanted to protect the peace and democracy in Europe. The position of the traditional leading powers of Europe like Great Britain and France were downbeaten by the War so they sought help in the one and only possible partner, the United States. The US first tried to keep distance from the problems of the Old World, but they saw a good chance to win new allies and keep the possible line of war away from they coast. They joined the coalition to fight Germany and helped to win the war. They liked how the situation was developing and to save the state and enable futher cooperation they launched several support plans like the Marshall plan and NATO.

Soviets chose a different approach. Proclaiming to kick "US capitalists" out of Europe and offer their socialistic dream to every country, they've found plenty of ears that were ready to accept that vision. Behind this friendly face the strong russian bear was lurking and waiting for the weak countries that were ready to accept his theories and later obey his commands when he liked it. In general Slavic nations feeled attracted by the new alternative which was comming from their "Big brother" and fell for the trick. With help from the communistic parties working inside soon the Eastern block felt confident and strong enough to refuse Western activities and signed a pact with the Soviet union.

Situation began to heat up, especially Germany became a hot spot. Stalin wanted to retain his main post in the middle of Europe and so did NATO. First ripping Germany into two states, then building up strenght behind the fences and walls. What followed was a harsh and bitter fight (thank God, fought mainly on political battlefield) for influence and governments. Neither party did accomplish their targets, but politics is not about winning but about making compromises. Europe was divided, split by the superpowers, more or less, dictating the rules to European states. The opposite sides were like two pitbulls sleeping at the different sides of a fence. They were running along the fence, watching, sometimes barking, sometimes even rattling at the fence, trying to scare the other dog away. We can be happy that no one of them tried to cross the fence and kill his opponent.

Ok, this is only a short and simple version of how I see it. The problem is far more complex and there is a wide variety of opinions to this question. The final answer will probably never reached, but that's the fate of historians.
My version is mainly built on the facts and is not going too deep to confuse too much. How you further explain the story to yourself is only your business.