This is a philosophically complex question that's been debated for centuries. Some philosophers argue that there's no truly selfless act, as even the most altruistic actions provide some form of psychological, emotional, or social reward. This ideological standpoint is known as "psychological egoism."
Others argue that selfless actions do exist - actions taken purely for the benefit of others, without any expectation or desire for personal reward. An action's positive byproduct, like feeling good about doing it, doesn't necessarily characterize the act as selfish, provided that the predominant intention was to benefit others rather than to gain personal satisfaction.
So it really depends on your philosophical viewpoint. Many people come somewhere in the middle, believing that while purely selfless acts may be rare, the positive feelings we get from helping others are just a bonus and don't negate the altruistic nature of the act.