Crystals have a lattice (grid) of atoms forming a consistent three-dimensional structure. The atoms are where they are because it releases energy for them to be in those positions. The surface of the crystal, the edge of the lattice, can incorporate new atoms and molecules on the side as the lattice expands, which is what causes crystal growth. Similarly, crystals can dissolve if it releases energy to take the atoms out of the lattice. In the case of most minerals, the atoms are free in a liquid state (either in magma or dissolved in water), so the cooler the magma or the more concentrated the water, the more potential there is for atoms to come out of solution and join the lattice.