This in itself is a difficult reason given many different theories. One that does a good job explaining is this one. (Please excuse errors and such as I'm on a phone).

Before WW2 there was a bi-polar power system in Europe. England and Germany? Correct me if I'm wrong as I'm speaking from memory right now.

During the war, many powers throughout the Allies and Axis powers had control over the world, including of course Europe.

When WW2 happened it destroyed all of Europe. Infrastructure was dismantled because of bombing and everything that comes with war, except for two. The "Untouched Exception", America, and the Soviet Union. After the war was over, a power vacuum was created. These now destroyed nations and defeated nations of the Axis powers needed to rebuild and start again.

To rebuild economies, these powers now only had two powers to turn to, capitalism within the USA, and communism within the Soviet Union.

Through countries needing help and the US and USES wanting influence, satellite wars contributed to the cold war, as well as containment and many other things.

Please keep in mind that answering a question like that requires a good knowledge that honestly extends before WW1 and well into the 80s and 90s even, which history hasn't truly reached yet. Studying that time period as a whole will also grant you with many answers to that question.

Also, please know that I'm no expert on this time period or in history at all, I just happened to have gained a somewhat substantial historical knowledge of American history throughout my college experience.

Edit: I'll be happy to answer questions on this as I can, as I love discussing this period of history. Once again, an entire university class could be taught on this subject, and A LOT of expansion on these points is required so any true historians please excuse my layman's explanation typed in a few moments on a phone. But I'll be more than happy to try and expand as I can and as wanted.