Part of the job of being a manager is disagreeing with other people.  

Look, their ideas may be wrong and you are the person who is going to  have to step up and let them know that they are wrong. As you can well  imagine, this does not always go over well. This is where arguments come  from. Arguments are a natural part of life. However, knowing how to  argue is one of the key manager skills that we need to have. 

However,  since none of us have had any manager training in how to do this, most  of us do it incorrectly.