Because everyone havs seven orifices, i.e. eyes, ears, nostrils and mouth. What is more important, each organ is connected with each other. When the nose is clogged, the moisture content streams out of your eyes. Hence,you realized your eyes water when your nose is stuffy.
It's because the nasal sinuses are close your eyes and your eyes and your nose are connected by the naso-lacrymal duct.
Your watery eyes can be caused by your stuffy nose. When you have nasal congestion, there is too much mucus inside your nose that some of it flows out from your eyes.
Well, I have to say that it can be very common to have watery eyes when you have got cold, and have stuffy nose. Generally speaking, watery eyes, stuffy nose can be the most common symptoms of cold. So you do not need to worry about it. And in my opinion, watery eyes can be caused by the stuffy nose, for nose is kind of related to eyes. So when there is something occurring in your nose, it will just affect your eyes, leading to some problems. For your situation, I will suggest you try some ways to treat your cold first, and then watery eyes can be fine by itself.