If self-interest is more "rational" than helping others, then you should be able to give good reasons for that with other words that are more clear and simple.

I think the gist is that we all live inside our own bubbles of consciousness and can only observe indirectly what is inside other people's bubbles. Everything that motivates you or makes you do anything is inside that bubble. If you expand this kind of thinking, it's not really important what is inside those other bubbles, only how they affect you. But this is kinda contrived philosophy.