Ungrateful people complain about the one thing you haven't done for them instead of being thankful for the thousands of things you have done for them. Meaning, ungrateful people do not care about the numerous things you have done for them. Instead, they would rather complain about the one thing you failed to do for them.
There are a lot of ungrateful people out there. They would rather insult you for the one thing you didn't do for them instead of thanking you for the plenty things you have done for them. So, you must be careful of the kind of people you help because you might decide to stop helping people if you come across an ungrateful person. Never let one person's character affect how you behave toward another.
You need to note that not everyone is ungrateful. Many people appreciate all you have done for them, no matter how small. However, ungrateful people do not care. This is why you need to be very sensitive of the kind of people you help so you won't be hurt at the end of the day. Never agree to help an ungrateful person if you are not emotionally strong.