I find it funny when people think of Ron as someone who values loyalty the least of the trio, when he clearly values it the most. Every single argument he has with either Hermione or Harry, in his mind (while not necessarily correct) is framed as a question of loyalty.
Don't go behind your friends' back: (A) telling McGonagall about the Firebolt, (B) Harry not telling him he got past the Goblet of Fire (don't get me wrong, I believe Hermione is right that he doesn't actually believe this)
Don't fraternise with the enemy - Harry is our champion and best mate, not Krum (yes, Ron... that's your issue with Krum and Hermione going to the Yule ball... my dear sweet teenage boy)
And of course the most important: the argument in DH. This I think geniunely is such a point of how loyalty matters to Ron. Ron's already inbetween a rock and a hard place. Ginny is clearly struggling with the breakup, however much Harry focuses on the fact that she understands why they've broken up. For Ron though, that's his little sister hurting. So he's a pain at the Burrow, but he never stops being Harry's best mate. He never truly blames Harry, but he does want to protect his younger sister. We often make Ginny close to some of the other Weasley brothers, but the two that most often show a protective streak are Ron and Percy. There are three people Ron truly looks out for: Hermione, Ginny and Harry, and two of these roles are, in his head, at odds.
So Ron's already struggling with this before they set off. Then of course we get the whole debarkle after Ginny is caught by Snape. Harry is dead worried about her too, we know that, but Ron doesn't see it because Harry does not remotely communicate his nerves, only his relief. On top of this, Ron's worried about Hermione, and how they're out there tenting without a plan. Importantly, in the books it's far less about jealousy (although that plays a part). "We thought you knew what you were doing." Not I. We. We've talked about it. We thought you knew.
“Take off the locket, Ron,” Hermione said, her voice unusually high. “Please take it off. You wouldn’t be talking like this if you hadn’t been wearing it all day.”
“Yeah, he would,” said Harry, who did not want excuses made for Ron. “D’you think I haven’t noticed the two of you whispering behind my back? D’you think I didn’t guess you were thinking this stuff?”
“Harry, we weren’t — ”
“Don’t lie!” Ron hurled at her. “You said it too, you said you were disappointed, you said you’d thought he had a bit more to go on than — ”
But first and foremost it's about Ron's family, which has suffered a fair bit, and how he question's Harry's loyalty to them (I mean, Harry literally throws Molly in Ron's face: "Did you think you'd be back to Mummy's by Christmas?")
“Go home then,” said Harry.
“Yeah, maybe I will!” shouted Ron, and he took several steps toward Harry, who did not back away. “Didn’t you hear what they said about my sister? But you don’t give a rat’s fart, do you, it’s only the Forbidden Forest, (...)”
“I was only saying — she was with the others, they were with Hagrid — ”
“Yeah, I get it, you don’t care! And what about the rest of my family, ‘the Weasleys don’t need another kid injured,’ did you hear that?”
“Yeah, I — ”
“Not bothered what it meant, though?”
"Ron!” said Hermione, forcing her way between them. "(...) I’m sure that’s all he meant — ”
“Oh, you’re sure, are you? Right then, well, I won’t bother myself about them. It’s all right for you two, isn’t it, with your parents safely out of the way — ”
“My parents are dead!.” Harry bellowed.
“And mine could be going the same way!” yelled Ron.
“Then GO!” roared Harry. “Go back to them, pretend you’ve got over your spattergroit and Mummy’ll be able to feed you up and--"
Ron made a sudden movement: Harry reacted, but before either wand was clear of its owner’s pocket, Hermione had raised her own.
“Protego,” she cried, and an invisible shield expanded between her and Harry on the one side and Ron on the other (...)
“Leave the Horcrux,” Harry said.
During the entire argument, Harry has to tell Ron to go multiple times before Ron leaves. It's not an easy decision, even under the influence of the Horcrux. It's not an easy decision, even though I am pretty sure not being needed is Ron's greatest fear (after, or possibly before, spiders), because who would really miss him as the sixth son--especially when his best friend is just saying: fine, leave, why don't you?
Even the whole "you choose him" shows exactly how Ron is my little Gryffindor first, Hufflepuff second, baby, because to Ron, the choice is personal. Hermione is making the choice based on the task ahead and the cause and keeping her word (and the fact that Ron's is the wrong) but to Ron, being in the right or wrong doesn't really matter. You have your friend's back, no matter what.
Ron wrenched the chain from over his head and cast the locket into a nearby chair. He turned to Hermione.
“What are you doing?”
“What do you mean?”
“Are you staying, or what?”
“I …” She looked anguished. “Yes — yes, I’m staying. Ron, we said we’d go with Harry, we said we’d help — ”
“I get it. You choose him.”
Hermione choses to stay, so thus she's choosing Harry.
So yeah, loyalty is the most important thing to Ron. We see this in his better moments too:
“That is the second time you have spoken out of turn, Miss Granger,” said Snape coolly. “Five more points from Gryffindor for being an insufferable know-it-all.”
Hermione went very red, put down her hand, and stared at the floor with her eyes full of tears. It was a mark of how much the class loathed Snape that they were all glaring at him, because every one of them had called Hermione a know-it-all at least once, and Ron, who told Hermione she was a know-it-all at least twice a week, said loudly, “You asked us a question and she knows the answer! Why ask if you don’t want to be told?”
Doesn't matter if Ron actually slightly agree with Snape, no one gets to hurt his friends.
Instead he contented himself with scrawling a note to Ron: Let’s do it tonight Ron read the message, swallowed hard, and looked sideways at the empty seat usually filled by Hermione. The sight seemed to stiffen his resolve, and he nodded.
Doesn't matter he hates spiders, he's following them into the Forbidden Forest if he has to.
This is the boy who sacrifised himself age 12 to help Harry get to the Philosopher Stone. This is the boy who stood up to Voldemort, a man he was so afraid of he couldn't use his name, at the absolutely darkest point in the battle--when everyone thought everything was lost:
“You see?” said Voldemort, and Harry felt him striding backward and forward right beside the place where he lay. “Harry Potter is dead! Do you understand now, deluded ones? He was nothing, ever, but a boy who relied on others to sacrifice themselves for him!”
“He beat you!” yelled Ron, and the charm broke, and the defenders of Hogwarts were shouting and screaming again until a second, more powerful bang extinguished their voices once more.
Ron is loyal, and he's self-sacrifiing (sharing his family which he already feels he's pretty invisible in with his famous best mate), and like all loyal and self-sacrificing people, they sometimes snap, because they feel like they're willing to give up the world for those around them, and they don't always feel recognised. Quick reminder, our boy got Fred and George to come help break out Harry at the Dursley's, age 12, because his best mate hadn't replied to his letters.
It makes perfect sense that it's Ron's lifeplans who change when George loses Fred, because it's in Ron's nature to put those around him first. If George needs help with the store, then Ron is going to help.
He's an effing god's gift to his friends and family, almost all of the time.
