Why Angular resets selected value?

I use select list in HTML:

<select name="mouth" ng-model="formData.mouth" ng-required="true" class="ng-pristine ng-valid ng-valid-required" required="required">
<option value="0">A</option>
<option value="3" selected="selected">Mart</option>
</select>

Angular resets selected value to firts option, when I added: ng-model="formData.day" ng-required="true"

What is a reason?

You should try using ngSelected. Make sure you are not setting the model value to '0' in the controller.