Why do particles and anti-particles, when they come in contact, annihilate each other?

For sub-atomic particles like electrons and protons, there exist corresponding particles with the same mass but opposite electrical and magnetic characteristics. For example, the electron’s anti-particle, positron, possesses the same mass as the electron, but carries a positive charge. The anti-particles are produced naturally in radioactive decay. In the universe, the number of regular subatomic particles and anti-particles is not the same; most matter in the universe is made of regular particles. When a particle and its antiparticle come in contact, both of them disappear giving rise to energy in the form of photons, gamma rays, etc. In such a reaction, if the particle and anti-particle pair does not annihilate itself, the sum total of mass energy before and after the reaction would not be the same, leading to a violation of the law of conservation of mass and energy.