The rudeness of gift recipients can be astonishing, sometimes. We both know someone who gets angry at the flowers her boyfriend brings her... because she doesn't like the ones he picks. He brings her flowers every other week, Just Because, and she raises her voice and complains that he doesn't listen to what she keeps telling him; that she can't stand daisies! Now, there are two issues at play here; 1) he doesn't listen, and 2) she's being rude.
You cannot fix the Giver. It's just not going to happen unless they are remarkably self-aware people who recognize their faults and work to change them. The only thing you can change is your own attitude. We're not saying to have a knee-jerk polite response to everything; there is a time and a place for gentle remonstrance. But that is remarkably rare - the majority of people have the best intention in the world when they give a gift, and you do not get to blame them if they make poor choices.
Susan has a friend who is a perfectly friendly, outgoing, and charming young woman... but who receives gifts as though they are a chore and she could care less about what she has just opened. She displays no enthusiasm and does not even say "Thanks." On the other hand, she has another friend who is prickly, easily riled up, and who openly hates getting presents because she doesn't feel like she did anything to deserve it... but is grateful and enthusiastic after she's opened them!
It is Courtesy, plain and simple, that we're advocating here. If someone has given you a gift, thank them cheerfully, and if possible, enthusiastically. If you know the minute that you open the gift that you will be taking it back for store credit the very next day, that's fine... but bear in mind that you must still be polite to the giver, and show appreciation for effort if humanly possible.