Rom. 13:8 ... "Owe nothing to anyone except to love one another."
The word translated "ungrateful" in IITim. 3:2 is acharistoi, which is formed of the root charis (kind) and the prefix a- (not). The idea expressed is that of not responding to kindness someone has shown in giving you something or doing something for you. There is something about receiving a gift that is seldom recognized. Suppose someone gives you $100 as a gift. What did you receive? Well, $100! But there is a lot more to it than that. How did the person get the $100 he gave? He had to work for it! Now suppose he earns $25 an hour for his work. That means the $100 he gave you really amounts to four hours of life in hard work. When someone gives you a gift, they are really giving you a part of their life, that is, the time it took to earn what the gift cost. No one can give you anything more precious to themself that a part of their life. When I receive a gift, that is what I think about, and it truly touches my heart very much. If it is cash, or a book, of a visit when I am sick, what I consider is the part of that person's life expressed in what they have given or done for me.
To show gratitude really does not mean to return something of equal value to recompense for the gift, because that negates the gift. If someone gives you a part of their life, what you really owe them is a part of your heart in sincere thankfulness. When I receive a letter or card expressing appreciation and thankfulness, it means a hundred times more than a return gift to "pay me back." I prize such written letters and notes and save them. I enjoy reading them again and again. To me the only thing that corresponds to a part of life someone has given me via a gift is a significant expression of love and kindness in a sincere statement of gratitude ... preferably in writing so that it can be read, reread, and truly "felt."