We accept both giver and gift into the circle of our life, allow them to be near us at least for a certain period, and perhaps even let them into the more intimate regions of our tastes, habits and feelings. This entails both respecting the giver’s intentions and learning discretion in our own gift-giving.
Thanking brings obligations just as a request does, and thanks should not therefore be given too casually. Through the expression of thanks we lift what is material onto a spiritual level. The giver’s thoughts are in the material gift often secret, often unconscious thoughts.
When the opportunistic motive is clear, accepting and thanking for a gift is already halfway to acquiescence in the dealing that will follow. To thank in such cases would rob us of our freedom.