It is human nature to feel uncomfortable receiving critics. So it is normal to feel uncomfortable receiving unexpected response from reviewers to a manuscript. However, it is more important to remind that reviewers are busy researchers who agree to spend time and effort to critic the work in their hand. And almost all of them do it for free.
The right attitude of the authors is to receive critics of reviewers with humility and gratitude. If the critics are done in a cordial and professional manner, the authors need to respond also in a professional manner, agreeing when the reviewers’ points are the right critics and arguing when the authors think that their points of view worth being reconsidered.
Time to time, there are lazy reviewers who gratuitously critic without presenting any arguments and without showing no alternatives/suggestions. It is then fair to respond, in an cordial manner, that laziness plays no part in the reviewing process. If one accepts to review, she/he agrees to be professional. And in such a case, it is extremely important to make clear to editors the poor quality of the comments.
Nguyễn Đăng Tùng 