The short answer is that in some cultures people will be offended if you refuse a gift. A friend of mine in college who was Venezuelan with a Syrian father would always take a stick of gum whenever one was offered, thank the giver and promptly put the gum in his pocket. He didn’t actually like gum. He was shocked when I said, ‘No, thanks’ to an offer of gum. He insisted that I always accept gifts graciously, and I always have. Exception: if the gift is extravagant, you should politely refuse, unless the giver insists too much, in which case it would be rude to refuse that new car.