The dog bites the hand of the feeder. A child screams angry on her mother. When a free service starts to ask for reward it's disregarded. When the hero has liberated the land, he soon becomes worn out and disregarded.
When the Buddha told that two people are hard to find in the world, one who does a kindness in advanced (without expecting something), and one who feels obligated to give back, it's not just a saying, but "brutal" reality under ordinary beings.
Gratitude, real gratitude, not just toward Mara (sense pleasure, which last very short) is something that arises only after winning the path.
What ever else gratitude toward Mara may come up and vanish, as fast as the benefit vanishes.
It's good to reflect about the topic.
Also to think about the reason of bearing ingratitude. The more one like to control someone, make him/her dear and own, win him/her for ones aims, the more one tolerates improper behaviour.
But there is also another cause that one is able to bear foolish behaviour of other: When knowing "beings are the heir of their actions", one focuses simply on ones own mind, speech, deeds.
As far as observed, a gift never really fails it's objected target. That means that it either works for a bettering of the receiver, or will have no benefit for him, slips out of his hand and finally meets a proper target according to ones perceived recipient.