Far from being bitter or haboring negative feelings, I would like to share my view on how to deal with ingratitude.
Like most people (if not all), I have experienced ingratitude from others in life including not a word of thanks when treating someone for lunch or giving someone a gift. I have 2 suggestions but am still pondering other solutions.
1) Calmly remind the recipient of your favor with confidence. For example, if the person did not say thank you to your gift, say “Btw, do you like my gift?”. If the person did not say thank you after you had paid for the meal, say “Btw, did you enjoy the meal?” No need to get upset. Stay calm and act confidently.
2) After a few such incidents of ingratitude from the same person, I suggest cutting your loss and NOT reinforcing such an unacceptable behavior. Say farewell to the person and his/her ingratitude.