I think TFA Prowl's self-restraint is more performative, and less an actual part of his personality. He enforces unecessarily strict limits on his self-expression (in almost all forms) because he's trying to imitate a virtue he admired in Yoketron.
But also because he hates/fears his own emotions, and the power they have to negatively affect his behavior and his life.
And also because fully experiencing his emotions brings his unprocessed traumas into the present - where he's desperately trying to avoid them.
It's no secret that Prowl has big feelings and strong convictions, despite his efforts to pretend otherwise. And in the flashback where he meets Yoketron for the first time, younger Prowl behaves almost exactly like current-day Bumblebee. He's brash, snarky, over-confident, condescensing, and feigns self-assurance as a means of self-protection. He also expresses his thoughts and feelings freely, without concern for how his words might affect others.
I assume that over the course of his training, Prowl came to realize how immature he was, and that his unfiltered self-expression wasn't a good thing. False confidence and hostility were signs of fear and weakness. True confidence and strength of character were marked by things like having an outwardly calm demeanor, or being considerate of others. Master Yoketron acted that way, and other bots treated him with immense love and respect.
Master Yoketron was also much more at peace with himself than Prowl was. Yoketron didn't struggle against his own emotions, and his sense of restraint was just an expression of his genuine self.
Prowl wanted to have that kind of control over himself. But he sought it through self-suppression instead of self-acceptance. And he loathed seeing Bumblebee act the way he used to - expressing himself without restraint, lacking consideration for others at times, and never having an outwardly calm demeanor.
Eventually Prowl realized that these traits didn't make Bee a bad person, nor did Bee deserve to be judged for them. And after learning the truth about Yoketron's death - and defeating the bot who killed him - I think Prowl did seem significantly more at peace with himself.
But he still hadn't accepted himself.
When Jazz affirmed that their master would've been proud to see Prowl wearing his recovered helmet, Prowl insisted that he would continue working to change himself. That he would make himself worthy of carrying Yoketron's legacy. Even though Yoketron had chosen and accepted him the day the met.
Prowl never got the time to process his grief for Yoketron, or his anger at Lockdown. And I'm sure he still had countless stellar cycles of ongoing trauma to heal from, even after bonding with Optimus and the others.
But he was finally making progress. He found the place where he belonged, and the family he belonged with. He faced (potentially) the most horrific trauma of his past, and came out the other side victorious.
And then some greedy chucklefucks at Hasbro decided to kill him off - completely unwarranted, with no narrative purpose for his death. (I'm not saying that flippantly; I wrote an academic-level essay on why Prowl's death in TFA makes zero sense for his character arc.)
TLDR: TFA Prowl's self-restraint and emotional suppression were a flawed attempt to be better than the person he once was. We were denied the beauty and catharsis of seeing him learn to accept himself while healing from his trauma.
















