The justification for this is reasonable, but doesn't cover all the motivations people have for good-deed doing.
The preachers decry the effort that some make to "buy their way into heaven" ― if I can call it that ― by hoping to build up a storehouse of credit for having done good in the world. The Pharisees, typically, thought this way. The Pharisees, with great self-satisfaction, gave money and did good things while carefully maintaining their exclusive upper eschelon status in the society of their time. I think the Ps expected a bigger mansion and a larger crown in the Land of Eternity.
[As an aside, I have very much the same problem with how the mechanism of karma is typically believed to work: Good actions get rewarded in a future lifetime; bad actions get punished, eventually. I don't believe that, and, more important, I think that focusing on trying to create good karma for oneself is a twisted way to live and be. From what I understand, knowledgable experts in these matters instruct that karma (if it exists at all) functions much differently that what is popularly believed.]
Anywho, getting back to Christianity and assessments there of good deeds. Christ taught that it is who we are on the inside that counts. And that our effort should be to better our inner selves, and not to try to impress God with activities that are demonstrably God-lovin' and people-lovin'. Altruism that is focussed on being a show of "what a swell guy am I" is uncool.
Ayn Rand with her Objectionist philosophy made a comprehensive argument against altruism, generally, that is compelling ― but also pretty damn spooky and cruel if imposed on all of humankind as a way we are to see ourselves and others. [It comports to the worldview of the Orange vmeme in Spiral Dynamics.] Basically, she believed that individuals have a right to act wholly in their own interest, but must recognize, and give sway to others, all of whom have the same unfettered rights as self-interested individuals. In her collection of essays "The Virtue of Selfishness," she wrote:
The Objectivist ethics holds that human good does not require human sacrifices and cannot be achieved by the sacrifice of anyone to anyone. It holds that the rational interests of men do not clash—that there is no conflict of interests among men who do not desire the unearned, who do not make sacrifices nor accept them, who deal with one another as traders, giving value for value.Hers is a belief in rational, unemotional Individualism. Hers is way of life that is spiritually retarding.
Human beings, unlike cockroaches, are social beings with a panoply of emotions. Also, truly, we do not exist as individuals though we have an "I" and experience the world as a self and act 'through" the body (that we drag around with us) and via the mind (that acts like a very unreliabe encyclopedia we can thumb through).
So, while thinking of ourselves as individuals (almost exclusively) and pursuing our own ends (almost exclusively) is pretty much the default way of existing, SEEING into our existance and getting fully in-touch with the reality of other sentient individuals being "just ourself in other clothing" brings a heartfelt, wonderful realization. It knocks the block out from under the delusion of individuality, and propels us toward altruism as something we must do ― for, as the Zennists say, no reason. Being altruistic becomes integral to who we are and to that which we've always been. [This comports to the Green vmeme in Spiral Dynamics. Green, btw, is just another color. And Spiral Dynamics just another way of organizing people's worldviews.]
Nonetheless, it must be said and known, that those who are determinedly self-interested are us [or, me, in my case] very very very much also. As are all the bank robbers and mother rapers and nose pickers and tax dodgers in the world. And as are all the peacocks and weasels and wolves and pterodactyls that there are or have ever been. And as are all the saints and holiest of men and women. And as are all the cutest of children and liveliest of puppies.
The fundamental delusion of humanity is to suppose that I am here and you are out there.
Yet, you must neither strive for truth nor seek to lose (or keep) your fundamental delusion.
We have two eyes to see two sides of things, but there must be a third eye which will see everything at the same time and yet not see anything.
The end of all our exploring will be to arrive where we started and know the place for the first time. We will feel as if we've been befriended by Alzheimer's. We will hope for and receive somebody to help us, maybe.
Doubt and chaos. There is nothing else. Frantic, we will search for a door. Any door. We might suppose that NOT being frantic is the answer, as we search for the answer ― or even THE answer ― instead of the door. We will get dizzy and fall. It is inevitable.
Suffering. So much suffering.