All molecules have London Dispersion forces, but both water and ammonia have particularly strong dipole-dipole interactions so that they can mix in any proportion. The strongest dip-dip interactions occur with F, N, or O with an H in asymmetric molecules. This dip-dip interaction is so strong it goes by a different name: "hydrogen bonding" (this is a dip-dip IMF, not the covalent bonding with hydrogen which is an INTRA-molecular force). This is why small alcohols also mix completely with water.
