Fusion is difficult to control because the reactions happen at temperatures of millions of degrees. A Helium nucleus consists of 2 protons and 2 neutrons. Two of the protons will be converted to neutrons in the process. Two protons thrown gently at each other will bounce off. If they get close enough, however, they will they will stick They must be thrown together at extremely high speeds.
High temperatures imply high speeds for gas particles.
The temperature at the center of the Sun is 10 million degrees. This temperature is just right to impart sufficient velocity to the protons so that fusion can occur. The final product, helium, is produced by a chain of reactions that gradually builds Helium from 4 Hydrogen nuclei. In the Sun this reaction is called the Proton-Proton Chain.