When people help you or give you something, they expect you to say thank you. It doesn't matter if the help or gift is unwanted.
Everyone you interact with is a part of a network that determines how you progress in life in very subtle but important ways.
If you fail to say thank you, people privately label you as unworthy.
Think of unworthiness as a really gross STD; it's the kind of thing people tend to bring up over and over again when you're not around.
You don't have to reciprocate every favor or gift. You just have to say thank you.