@CKN I think although there may be some partial exceptions (Nora, although she is still childish in a lot of ways as well), by and large the mindset of shinki are stuck in the age that the died. They may gain new knowledge over time but it can't change the way they process the information. Remember Kofuku's child shinki? He kept asking why he didn't grow older when all his friends did, because he died too young to be able to comprehend concepts like death or immortality.
So even though Yukine logically should know better, he, as an eternal 14yo, can't help the fact that he reacts impulsively and emotionally, especially compounded on his emotional trauma of abandonment and abuse. He's tried to keep his feelings in; he was a good boy for such a long time after his initial stealing drama. He studied hard to be a good hafuri and to learn more as a student, even though he has no 'future'. He trusted Yato, despite him flaking out for months at a time and never telling Yukine anything important. Then, despite all his efforts and all of Yato's praise, he sees Yato apparently abandon him for another hafuri. Actions speak louder than words; all of Yato's praise seems fake when he shows Yukine that he prefers a 'more powerful' shinki over him. He's a teenager, he can't help acting on his emotions rather than rationally thinking it through.
idk y'all are being way too harsh on Yukine when his characterization's been consistent throughout the manga, he's always been a central part to the plot, and his backstory has always been an issue waiting to be addressed. I'm content to just be patient and see through the end of the arc because the authors definitely have a plan and I don't want them to overwork themselves to point of injury again to please impatient readers.