First of all, please note that all four Berlin zones did not join together, at least not until Germany reunified in 1990. The zones administered by the three Western allies (the US, Britain, and France) did unify in 1949. They did so because the Western Allies wanted a stronger West Germany to help protect against communism.
The Western allies were very concerned about the possibility that communism would spread across Europe. They believed that "their" parts of Germany would be much more likely to sympathize with communism if they continued to keep those zones separate. They felt that putting the zones back together would make Germany stronger economically and make Germans happier to ally themselves with their former enemies. The US, in particular, pushed for this to be done.
The USSR did not want this to happen and blockaded Berlin to try to prevent it. The Allies then mounted the Berlin Airlift. Soon after the Airlift caused the end of the blockade, the Western allies joined their zones in Berlin and in West Germany.