I read somewhere, you get maximum happiness when you help someone who is not in a position to help you. Seems to be so true.
When you help someone who is in a position to reciprocate, expectations come into play immediately. You make a mental note of what you have done for the other person and expect him to return the favour at the earliest. The expectation interferes with the intensity of the bliss you experience.
The expectation may itself become a source of misery rather than bliss. Non-fulfillment of the expectation haunts and torments you. Sometimes, the other person may not have even the slightest idea that he has become a source of misery to you.
When you help someone who is not in a position to reciprocate, you are free from any baggage. You help because you want to, and not because you are going to get anything in return. There is no expectation and no associated pain. You know that you are helping the most deserving. When the other person gives you a smile, it becomes your biggest reward and you start experiencing true bliss.