It is because the first time you do something for someone, you create a feeling of gratitude.

The second time you do or give something to someone, you create expectations. The person expects...

By the third time, you've already created a sense of waiting. The person is waiting to receive...

The fourth time you make one feel valued. The person feels worthy of what you give and wants to continue to receive.

By the fifth time, you have already created an addiction. This person feels he can't live well without what you give them. You have already spoiled them.

The sixth time you realise that there is no reciprocity, and you stop giving, then the spoiled person you have created will be angry at you for denying him what he so desperately wants, and will end up hating you because you have stopped giving him what he thinks he deserves.

That's why you have to put a LIMIT on giving, because the other person has no limit on receiving.