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I think your view is based on a nihilism which views the mental as "mere material stuff" and concludes that because it doesn't matter to rocks and sand what we do to rocks and sand, it ought not matter to us what we do to ourselves. Bad logic really. Were a different species of entity.
"Mattering" is wired into us insofar as we have feelings and needs, and also as we live in relation to an external world which can be organised either benignly or detrimentally also. Its "rational" to choose that which benefits, as its rational to choose a better option. Its a potent logic that is appropriate to the state of affairs we're faced with.
So some form of rationality is our purpose, I call it 'rational attraction to being' and it comes on a sliding scale - it can occur either consciously when we reflect on then big picture, or in general everyday affairs like eating, sleeping, chatting online etc. but in a more pre-reflective fashion.
So rationality is a kind of mechanism, but also a pre-given project at the same time. The "being" bit, that's what our rationality relates us to, and maintains us in, for better or for worse according to our skill. (cf the "sliding scale" idea).
Its also not all vain because its all passing by, that's more bad logic. In fact without change and time we couldn't be purpose minded creatures with projects, adventures and quests, or nights in etc. So therefore change doesn't destroy purpose and meaning, its actually a pre-condition of it!
You cant make a sand castle (or a home) unless you're in a partner the space time continuum, yet the essence of 'raitonal attraction to being' remains as a mechanism and a goal.
Hope this helps.