The arguments that point in favor and against altruism are based on the principles of motivation: since all of our actions are carried on by the amount of motivation we place on them, then the fact that there is a degree of motivation alone rules out that altruism could be possible.
However, there are also counterarguments to this posture which clearly indicate that it is false to assume that the presence of motivation alone should be considered a rationale for wanting to do good onto others.
Altruism is the "unselfish concern" for the welfare of others. It entails that we feel and do onto others without expecting a reward because we have come to a point where we love ourselves enough to understand how to love others as well. That is the ideal definition of the term. Therefore, someone who claims that altruism is not possible will perhaps say that the love for others stems from a self-love which very well may translate in some cases as narcissism depending on the situation. For example, there are people who get into philanthropy to do good onto others and expecting nothing monetary in return. However, the motivation does not stem from getting financial gain in return, but from getting a chance to tell a story about themselves, or from placing themselves in a sort of super-hero position.
Although these instances are realistic, the truth is that they do not comply with the true meaning of altruism. Therefore, they are not false acts of altruism that would lead us to conclude that altruism is not real at all. They simple are not altruism. They are something else.
True altruism, as the real term entails, is quite real. From the perspective of Maslow's Hierarchy of Needs model, for example, altruism would be a top behavior that comes as a result of self-actualization. Self-actualization is that "self-love" that people who do not believe in altruism doubt about so much.
Yet, self-realization does exist. It is a process that a lot of people never get to experience in their lives. Those who do experience self-realization are people who accept themselves, their limitations, accept their strengths and understand the paths of their lives with clarity. As a result of learning something so complex, they are able to help others reach that same level of awareness. This, they do with no intention of gaining anything back: they are individuals with a higher consciousness who merely help others get there too. This would be a very valid argument in favor of altruism, and the reason why there is no doubt that it does exist. Altruism is a very hard-to reach emotion that comes from a very hard-to achieve personal goal.