Why is it dangerous to add water to a concentrated acid?

Diluting concentrated acid with water causes a great deal of heat to be generated. If the water is added to the acid the water can reach its boiling point causing an explosion of steam, which can spatter acid all over. The effect is especially dangerous with sulfuric acid, but it is easier to treat all acids with respect and not have to remember which ones have this problem and which don't.
If the acid is added to the water the heat generated can be distributed through the water quickly enough that the water never reaches its boiling point and the likelihood of spattering is much less.

The reason is because acid reacts with with water vigorously. you can search google for well defined answer.

it will bubble and smoke, possibly hurting you in the process