In business key is to look for problems and offer right matching solution to grow your business. So when any client complain, you have new opportunity, do you agree?

Strongly Disagree. If a client is complaining that means the key is missing.As said by you the key is to look for problems internally and that should not be pointed out by the client.Being in a business we will have complains / solutions but that doesnot seems to be an opportunity.