When there is a deepening transdual view (first through awakening as the Witness and seeing the world of phenomena as a seamless fluid whole, and then as "no I" even there and all of Existence beyond all polarities), there is also a deepening sense of Existence as "one body" - beyond and including all polarities.
And from here, a natural and effortless compassion emerges. Just as I pull my hand away from fire, I naturally engage to alleviate the suffering of others. This hand, that human being, it is all part of this same body.
And this is from a direct and effortless experience of what already is, not from any abstractions or ideas of "one body" or any "shoulds" associated with it. That is indeed why is is effortless, untiring, natural and spontaneous - to the extent it is not clouded over by beliefs in thoughts.
Self-interest as altruism
When we exclusively identify as a human being, our natural and effortless self-interest sometimes manifest as narrow selfishness.
As we mature as human beings, we tend to expand our identity and our circle of concern. Now, we may for instance include our family, friends, community and country. And still, there are large parts of Existence excluded, which is how it should be. It is just a natural part of our maturation process.
Eventually, as we awaken as what is - as Existence in human form and the view of Existence as a whole - our identity and circle of concern continue to expand and become more inclusive, until any boundaries and exclusive identities fall away entirely.
Here, our natural and effortless self-interest is revealed and expressed as natural and effortless compassion for all beings and all there is. It is all part of Existence, of this One Body. And it is a direct realization, it is fully lived in a deepening way, not just by trying to believe in abstractions.
It comes from simply awakening as what already and always is, as it is.
So our effortless self-interest does not really change, it is just what we experience ourselves as that changes, and that makes all the difference.