Chemical Reactions and Equations
This is a HUGE question to answer completely!
One answer is 'because they result in a negative change in free energy, delta-G.' This may be as a result of the reaction being exothermic, so the products are more stable than the reactants, or could be a result of an increase in entropy (products more disordered than the reactants), or both of these.
Another answer is 'because their activation energy is sufficiently low' so that successful collisions between the reactant particles can take place.
If you can refine your question a bit, make it a bit more specific or place it in a context, I can have a go at explaining in a bit more detail, unless this suffices for your needs.