My understanding of "self" is that it's an artificial and transient construct. It comes and goes. My theory is that "self" evolved as someting utilitarian for the sake of interacting with a complex physical world; it exists as a relational referent for navigation and survival strategy.
There are moments when you're a "you" and other moments where self is absent. We know this because when we meditate, self disappears. Therefore, meditation is a selfless act. So is sleeping.
Maybe people have a hard time understanding selflessness because we live in a degenerate time where semantics, logic and pure rationality are used to justify everything, and selflessness is not rooted in logic. A creative person doing an abstract work may have no method at all, or no inherent selfish desire. They simply channel creation.
Have you ever watched yourself doing something, and you don't even know why you're doing it? Where is self in that moment? Or better yet, why does self (ego) lack awareness in that moment?
Whether or not you believe in selflessness comes down to whether or not you believe the ego-based self is real and the be all and end all of perceptual reality. My experience of self (and ironically, I am "self" talking as I say this), is that it's transient to such a degree that it can only be a program like all the other thought streams happening. None of them are the real you. If you believe mind is real and the only aspect of consciousness, then selflessness cannot possibly exist because self is all there is.
If you find a calm space and quiet the mind, it will be evident that there's no self in there. And while I am not an enlightened being, I can surmise a level of spirituality where one is closer to pure action than mind, and is thus capable of many selfless acts; and not only grandios ones, but mundane everyday tasks.
A good example is a baby sitting in the middle of the road with a car heading toward it. Many people's desire to save that baby is going to be based on instinct, or, at a higher level, compassion. There is no mind involved there. There's simply action, as there's no time to think about all this semantic crap. You just act and that's that.
A heart-centred person living in a compassionate consciousness does things just for doing them, and not because of any reward. But it's going to be difficult to convey this to people not in that consciousness, because their reality is still based on a clingy self that makes constant, irrational demands. In other words, selfishness. Ayn Rand succinctly describes the overwhelming majority of humanity who do NOT cultivate heart consciousness. We may see glimpses of it, like in how people interact with their children, but as an over-arcing consciousness that coordinates entire lives? Most people just aren't there. They haven't done the necessary inner work to overcome fear, sex drives, and emotional baggage... thus their motivations shall remain selfish, and thus humanity as a whole remains in a suffering paradigm.