the lion chooses its paladin and forges a mystical bond with them. these lions have already chosen their true paladins. their choices for new paladins are for the sake of their true paladins.
in s2e1, black only responded to keith because shiro was in danger and keith could save him. in that same episode, shiro told keith that he wanted him to lead when shiro was gone.
in s3, shiro was gone, and although allura is the “decision-maker”, black chose keith.
it all comes back to the trust between black and shiro, something that grew between them throughout two seasons. it’s the foundation of their mystical bond.
black has been used and abused by zarkon, but shiro respects her and knows that she cannot be commanded. this has earned black’s trust. if shiro truly believes in keith as the leader of voltron, then black will trust that.
EDIT: as an anon pointed out, keith was talking to shiro when black chose him, making it even more obvious that black is carrying out shiro’s wishes, not her own. black literally responded in shiro’s place here, trying to tell keith, as shiro would, that he can lead the team.
lance even said, “i respect its choice” when black chose keith. by extension of respecting black’s choice to trust shiro’s judgment, lance and keith are trusting shiro’s judgment as well. the very thing that keith doubted so much.
furthermore, this would add significance to black rejecting the shiro in season 3. she didn’t reject him because he was unworthy or because she’d moved on; she rejected him because she does not trust him.
TLDR: the mystical bond between black and shiro is founded on trust. black chose keith because shiro chose keith, and she trusts her true paladin’s judgment.
red called out to lance only after keith literally called out to lance for help. essentially, red called out to lance because keith called out to lance. her desires mirrored keith’s. if keith wanted lance to be there, then by god lance would be there.
get in, lancey lance, we’re going
shopping to save my son.
throughout s3, lance looked out for keith both physically and emotionally, more so than any other character. without his help, it’s not fun to imagine what would have happened to keith and the rest of the team.
red has demonstrated on two major occasions that she is very concerned with keith’s welfare and will do anything to keep him safe. this is particularly interesting, since she was so temperamental when she first met him that she waited until he was about to die to let him in. the foundation of their mystical bond is protection.
we know that lance in particular is great at keeping keith safe, even if he wasn’t before… (hmmMMMMMMMM. HMMMMMMMMMMMM.)
furthermore, allura emphasized that red must have chosen lance to be keith’s right-hand man, to which lance said, “i won’t let him down.” he explicitly made the connection between red calling out to him and keeping keith safe. this is red basically saying, “please keep my idiot son from getting himself killed. be his impulse control, i believe in you.”
TLDR: the mystical bond between red and keith is founded on protection. red chose lance because he will keep her true paladin safe.
lance said it himself: “i’m glad it was you.” it’s as simple as that. despite his obnoxious flirting that he’s done in the past, he deeply admires allura. this dynamic in particular really appealed to people, but it was a new angle that had yet to be explored until this opportunity arose.
blue must have know that lance being separated from her would be difficult, but she guessed (correctly) that allura taking his place would put him in a position in which he wants to be supportive and feels the least upset about it. despite all my complaining about blue shutting him out, i think this decision was made with lance’s self-doubt (which is exactly what blue responded to in allura) in mind.
blue supports lance. she’s his best bud, his old girl, his blue, his beautiful. she makes him feel useful and happy. as the “mom” of the lions, she’s there for him. the foundation of their bond is support.
TLDR: the mystical bond between blue and lance is founded on support. blue chose allura because blue knew that lance would be supportive of allura.
as for why lance needed to lose blue at all, maybe she knew that red would continue to refuse allura (being temperamental about keith), and thus needed to force red’s hand. this one is still…kinda fuzzy as to the “why”, but the “who” absolutely matches up with the pattern. that fuzziness also lends itself to a potential explanation for blue being so…cold about the whole thing, both to lance and to allura.
black’s true paladin is shiro, red’s true paladin is keith, and blue’s true paladin is lance. the lions chose new paladins for their respective true paladins, not for themselves as they did initially. this is merely emphasis as to who their true paladins are.
it’s interesting that lance was the one to confirm the true reasoning behind all of these switches. black: “i respect its choice.” (respect shiro’s choice to trust keith.) red: “i won’t let him down.” (protect keith.) blue: “if i had to lose blue to someone, i’m glad it was you.” (support allura, thereby supporting lance.)
this highlights why season 3 was so effective in strengthening the bonds that lance has with both allura and keith. the switches were based on the bonds that their true paladins had with the other paladins, so of course the switch would strengthen those bonds. lance and allura always had the potential to mutually respect and admire one another, and now they do. lance and keith always had the potential of being leader and right-hand man, and now they are.
TLDR: lance’s bond with keith was strengthened because red chose him to protect keith, her true paladin. allura’s bond with lance was strengthened because blue chose her to support lance, her true paladin. black chose keith to trust shiro, her true paladin, and rejected new shiro because she didn’t trust him.
if this is the explanation, then i’m honestly okay with the whole thing.