There are four fundamental forces in nature. They are the strong nuclear and weak nuclear, electro-magnetic, and gravity forces. These all work with and against each other as the universe tries to gain a stable, low ordered state (lowest energy-most random). you need to realize that matter is another form of energy.The center of an atom, the nucleus, is held together (work) by converting a little of the mass of the particles of the nucleus into a binding energy. This is needed to keep all those positively charged protons so close to each other. For light elements, if the number of protons and the number of neutrons are the same, all the forces acting in the nucleus are well matched and the nucleus is stable. But if there are too many neutrons or protons, then the nucleus has too much energy and will normally transfer energy around until the 1:1 neutron to proton ratio is achieved. This frequently is seen as the emission of the energy, or what is called radiation. At higher atomic numbers, there are so many protons, that you need more than 1 neutron per proton to hold the nucleus together. However, there still may be stable configurations for the atoms, and the atoms may try to reach those states by emitting the larger alpha particle. Sometimes, following the initial release of energy, there still may be extra energy in the nucleus, and this can be emitted as a photon, or by transferring the energy to the orbital electrons.
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Some are born that way and some are impacted by outside stimulus. Radioactive decay can be a powerful scenario in many applications. It is not a bad thing.