Fighting.
It used to be a thing of joy for Damien.
Tip-toeing on the tightrope of death, knowing his life could slip out of his control at any moment, that feeling used to excite him more than anything else.
When he grew out of his insanity, he still kept his love for battle, but its meaning changed. Instead of a joy, it became a necessity.
He fought with purpose, so that he could accomplish whatever he needed to accomplish at that time. He also fought for survival, purely trying to live as countless threats tried to make sure he didn't.
These motivations dulled his joy, because he no longer had the leeway to have such lighthearted thoughts.
Such was the burden of adulthood. He was no longer a youth who could do whatever he wanted with his life. He didn't have something like a support system that could cover his mistakes for him.