Atlas had been under quite a lot of stress since he believed he had to face Atlan and Yovana. He was in a race against time before. But now that he managed to retrieve the stone from Atlan, his stress went down quite a lot. Now the only opponent that he had was Ramiun, which was comparatively more relaxing.
He had dealt with all the Progenitors. Now all he needed to do was deal with someone who was strong but not on the level of a Progenitor, and he had quite some time for that. He still wanted to finish the task at hand as fast as he could, but he could also relax for now, take a few moments to breathe, and have some fun.
Because of what he went through, he was forced to grow up and mature fast, never being able to live his childhood. But now that things had started falling to place, he was starting to recover his old self that he barely managed to experience.