We have to use warm water when making yeast dough because- The dough needs to be warm so that the yeast can activate and start growing.  Yeast only activates properly while you use warm water or milk. Note that- If your water is too hot, it will kill the yeast. If your water is too cold, the yeast will not be able to grow, active, and enough to do its job. The ideal temperature for activating yeast is between 105 and 115 degrees Fahrenheit.