When NH4NO3 spontaneously dissolves in water the temperature of the solution decreases. What factors make this a naturally occurring process?

Once solid ammonium nitrate interacts with water, the molecules of polar water intermingle with these ions and attract individual ions from the structure of the lattice, that actually will break down. E. g; -NH4NO3 (s) - NH4 + (aq) + NO3 - (aq) To split the ionic bonds that bind the lattice intact takes energy that is drained from the surroundings to cool the solution.

Some heat energy is produced once the ammonium and nitrate ions react with the water molecules (exothermic reaction), however this heat is far below that is needed by the H2O molecules to split the powerful ionic bonds in the solid ammonium nitrate.

Hence, we can say that the dissolution of ammonium nitrate in water is highly endothermic reaction.