Also most definitely overthinking, but the Tibarn and Reyson A support in Radiant Dawn is VERY interesting.
Tibarn straight up "ordering" Reyson not to die. Reyson just smiling at him and going "I won't. You've made me so happy."
Reyson. What did you mean by that...
Tibarn is pulling the "I'm the king" card here, but his order is no different than the bunch of desperate pleas so many people in RD make in their A supports, which is particularly dramatic for those who only get the chance to A support at the Tower of Guidance, they genuinely think they might die there.
Reyson either knows this and understands the emotions that are leading Tibarn to ask him to live, which is why he brings up how Tibarn has made him feel or...
We know Tibarn was the one to physically rescue Reyson during the Serenes massacre and look after him after that. Now, obviously this was an incredibly traumatic experience for Reyson, the hawks were only able to save him and his father, everyone else is gone. And his father wasn't doing particularly well, especially mentally speaking, so he wasn't even a source of comfort for Reyson.
I can imagine Tibarn ordering him to live back then because Reyson wouldn't find many reasons to want to, with his family gone, his nation gone, his forest gone. There's that sort of pact between them, with Tibarn taking revenge on Reyson's behalf, after all. I get the feeling Tibarn might have told Reyson to live, might have promised he would show him there were reasons for him to and that he would show him so one day he could be happy again.
The laguz live a LONG time. It's been around 20 years since the Serenes Massacre by the time Path of Radiance comes around. It's a hell lot of time for Reyson to have lived full of resentment and hurt, taking solace on the revenge Tibarn is taking in his place, but overall perhaps not that long for a laguz.
Now, there's only 3 years between Path of Radiance and Radiant Dawn, 3 short years since Reyson has changed his perspective and found a new reason to live, both because of his realization that the beorc can be good and because of Leanne. I wouldn't be surprised if Tibarn is still worried about him.
So a plea, "live, I still want you to live, please", and a response, "you've made me happy, I don't want to die anymore".