Hume made his 'Is-Ought' distinction exactly because he saw reason alone as unable to provide impulses to act, we must rely on our animal passions for these, our evolved need to sustain the causes of our arising.
The utilitarians proposed a strict principle of maximising pleasure or happiness or some other measurable verifiable non-subjective goal. That leads to problems like Nozick's 'utility monster' https://www.smbc-comics.com/comic/2012-04-03 and, well to many other problems to list.
I hope this helps, and at least points to topics that may interest.
The other argument against a creature of 'pure reason' is the Private Language argument, developed from Wittgenstein's ideas. It devastates Decartes' idea of a pure subject able to reason without the implicit assumption of reason users to share the symbol use required for abstract reasoning with. Increasingly cognitive science is pointing to the intelligence of emotions, see for instance the book Thinking Fast And Slow, whuch outlines reasoning as for slow 'safe' situations, emotion and intuition as best guess and 'dangerous' situations. In this view emotions relate to the wider frame, including survival, whereas reasoning can choose to put all kinds of wider considerations on hold - but in terms of understanding the future the wider context will hold. Pure reasoning could arguably get there or choose to, but would perhaps need to regenerate emotiins for heurustic rules-of-thumb.
If the motivations of a human were completely known from a system or framing, they would be recursively enumerable, or Godel-complete. That is, fundamentally unable to adapt beyond a certain point and recognise new truths. We observe that this is characteristic of all known synthetic intelligences, but not of human minds. It seems 'free will' is exactly the ability to act irrationally within any known system, by stepping outside of it and creating a new system with the old one as a part. So recursion, self- conceptualising, or more colloquially: are self-conscious.
I hope this helps, and at least points to reading that may interest. Welcome to SE-Philisophy.