A man saving his dog from burning fire, a women rescuing a drowning person from a river we have all seen the headlines before of the selfless act of heroism. But in truth everything done from going downstairs getting breakfast to building schools in Africa helps you in some way. I am sure whoever is reading this has heard the phrase "every action has an opposite and equal reaction" in this case those selfless acts become self-helping acts. Let me explain "police officers risk life and limb to stop drug cartel" seems heroic no? Well it isn't like the police officers do not gain anything from doing so. By stopping the drug cartel they are making the neighborhood safer for everyone including them. Of course that was a weak example but I think it proves a point. Everything is to be benefited from if not directly to you then to the things important to you which should be accounted as a part of your self-interest.
The only way to not operate of self-interest is to not operate,
Humans are selfish creatures. However, humans as a whole are very social creatures. In nearly every situation, it is more beneficial to ones self to benefit society and your reputation within it rather than a quick solo gain. Evolution has shaped us to feel empathy, sympathy, anger, etc, so that we can keep each other in check. Devoting 100% to others will result in your early demise. Being selfish and stealing or freeloading will lead to community backlash, leading to your early demise.
However, some individuals genuinely have compassion and empathy for others and may risk their safety or survival security in order to assist another. Most humans contain an ulterior motive and progress forward merely to benefit primarily themselves. Whenever possible I attempt to help another in any fashion that I am able.
The natural human is communal. Independence in society is a norm born of an overabundance of skills but in smaller tribes and villages or circles you still do what is best for the group based on your personal abilities, skills, and talents regardless of how you feel about the matter.
It is possible to boil down even the seemingly most generous act as being psychologically self-serving (i.E. I give money to charity so I can feel good about myself). The answer as to whether or not an act is self-serving must come from an understand as to what the initial impulse is (conation). If all impulses in humans are selfish, then this opinion would be true. Being that such a statement is a generality, it is not possible to outright claim that all impulses are self-serving. I don't think it is possible to prove or disprove at our current level of understanding of the human psyche.
I could've easily gone with either yes or no for this one, but I chose no for the reason that humans don't operate SOLELY on the base of self interest. I agree in that things humans do have an underlying interest for personal gain, but it is not the SOLE reason we do such things. I'll be one of the first people to admit that I haven't done something purely out of selflessness, because everything I've done for others has benefitted me in some way. But I don't do these things SOLELY because it benefits me. My personal benefit has been at the back of my mind, but it's not the reason why I do it. So no, humans don't operate solely on the basis of self interest.