To appreciate something is to understand something well enough to know why it is relevant. If you are not aware of the existence of a person, how can you appreciate them? If you have no knowledge of what they have done or you do not understand why they did it, then you cannot see why it is relevant to you.
To appreciate something also implies that you are thankful or grateful for it. I think that the reason people are ungrateful is because they do not understand the reason why something is given to them or why someone did something. Maybe they simply haven’t had time to think about something long enough to appreciate it.
At the same time, understanding something can make a person appreciate it even less. If someone gives me a gift, I might appreciate, but if later on they tell me that I have to do something for them to return the favor, then their gift has become irrelevant because I knew they were just deceiving me so that they could get something back.
Giving only to get something back is not giving. When you give something to someone, you have no right to take it away just because you didn’t get the reaction you wanted. When someone does something for the wrong reason, I do not appreciate it because it has become irrelevant.