Well, it's because all of the water collected in the cloud is pure water and pure water can only freeze when it sticks to a particle, such as a particle of dust. So, as the frozen pure water continues to build up on the particle it makes a snowflake. "Depending on the conditions, different shaped crystals will grow.
In general, we see that snowflakes have a six-sided symmetry. This is because of the shape of a water molecule (two hydrogen atoms and one oxygen atom stuck together), the angle between the two hydrogen atoms is about 120 degrees. This is also the angle between two sides of a regular hexagon (six-sided figure where all the sides are the same length). So in a sense, water molecules like to "fit together" in six-sided shapes."
Quote and information from http://www.faqkids.com/idx/4/082/article/How_is_snow_made.html
Quote and information from http://www.faqkids.com/idx/4/082/article/How_is_snow_made.html
There is also another factor that contributes to the formation of snow. The troposphere has to be below the dew point AND the dew point has to be below freezing for snow to form. Also, the 'dust particle' is actually called "cloud condensation nuclei" and us only used when liquid drops are formed. When the clouds are cold enough to form frozen drops, or snow, then the frozen particle acts as the nuclei needed to make it freeze. So, snow doesn't need a CCN (cloud condensation nuclei) to freeze but it does if it isn't going to freeze.
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