He had waited so long for Kael'thas to come back. Lor'themar had based his strength on the enexorable fact that Kael'thas had to return to them; that his Prince would return and release him from the heavy burden of the Regency. Kael'thas had returned, wreathed in flame, with the whispers of atrocities in the Outland following at his heels, but he had returned. They whispered of deals struck with the Burning Legion itself, of terrible things.
Some Lor'themar knew were true; he himself had demanded answers when Kael'thas had suddenly returned after being gone so long without a word. The deal with Kil'jaeden had been a shock, that they would be forced to resort to such lengths to restore the Sunwell leaving him ill at ease. But his Prince had returned with a true solution to their plight; his Prince had left him to defend their people because he knew that Lor'themar's loyalty was unwavering; his Prince had asked for his loyalty once again, to restore their people and ensure their future security. Perhaps his loyalty damned him, but he followed his Prince.
Kael'thas had told him about the attempt on his life at Tempest Keep, and the necessity of their haste in arranging the ritual that would re-ignite the Sunwell, but having what needed to be done described to him and seeing the frightened human who was supposedly the avatar of their Sunwell's remaining energies being led toward M'uru's serenely floating form was something else entirely.
He was privately glad when it was over, the now-fading forms of the N'aaru and Anveena blending together into the bright glow of the Sunwell. Lor'themar can feel when it fully re-ignites, the wash of light and warmth reminding him of all the things he hadn't known he was missing.
Afterwards, all of them buoyed by the light, Lor'themar can hardly remember their journey back to the city. There are flashes of celebrating people; the whole city honoring their Prince, their savior. He tries as best he can to step to the side—he was only a stop-gap, holding them together until Kael could return—but his Prince continually drew him back.
It was only much later, when he was somehow alone with his Prince that all that had happened caught up to him. The rush of the Sunwell's re-ignition almost as great as the rush of having Kael'thas home again. More of the celebrations filtered back to him; Kael'thas keeping him close; Kael'thas smiling; Kael'thas taking hold of his wrist and leading him away from the joyful crowds filling the streets.
When Kael'thas pulls him close and kisses him, whispering that together they would see their people safe and proud again, Lor'themar sighs his relief into his Prince's mouth.
Notes:
This did not turn out like I expected, but still, in canon all I imagine Kael wanted was to fix the Sunwell and screw over Kil'jaeden. Those pesky adventurers just got in the way