WIY, the reason why we are not MECHIOV to do chesed for all people we come in contact with and might need our help, is beside that we’ll never be finished, it is humanly impossible to give more than one can.
If you can give chesed, fine. If you can’t give in certain situations, the Torah does not obligate one to do so. Even with parents, if a child cannot care for them for a certain reason they need to pay for someone to take care of them, but they don’t have to do so themselves.
Now while one needs to understand that a thank you and appreciation might not come their way, but if they are constantly taken advantage of and they feel burnt by such a relationship, then they are not mechiav to be in such a relationship and not mechiav to perform chesed for such a person either.