You get freezing rain when the temperature high above the ground is above freezing, and the cold rain falls on surfaces that are below freezing. More about why it rains, or snows, or sleets, or hails. Click to enlarge.
Water needs a nucleus, or core, upon which to form crystals.
If the snow falls through air with a temperature below freezing, it'll stay snow. If it falls through a deep enough layer of air that's above freezing before it reaches the ground, it'll melt and become rain. Once it melts, a raindrop can't turn back into snow, but it can re-freeze into ice. In fact, sleet happens when raindrops fall through a layer of freezing air that's thick enough for them to re-freeze before reaching the ground. Meanwhile, you can get hail in thunderstorms where there are strong updrafts and down drafts.