There are times you will get surprised by requests made by people you know. Sometimes they are difficult to say yes or no to immediately. Sometimes you will have a gut reaction one way or the other.
Rarely should you give in to the gut reaction. If someone has given a topic a considerable amount of thought and believed it was worth raising as a question, their view should be given appropriate consideration. Saying yes or no too quickly can diminish the request.
An open mind is important as it lets you go away and consider the request independently. It also gives the opportunity to seek outside thoughts or even take the conversation further for more information.
Changing a yes to a no or a no to a yes can often be difficult after the fact.