Since way before in his first life, he was naive. I mean, he lost all hope. clinging only to a single person, Irene. That's already pretty naive. Its even showed that he fight through 100 soldiers just to find Irene. Not that I can blame him with all that trauma. The only way to fix a Naivety is...
I believe the "Loner" here is just a nickname, like an "insult" you call to an always alone person. I think the right description for him is Introvert. And you probably won't call someone Introvert as a nickname right?
For people who said training for 100 million years, but still have the naive mentality. like @dend08. I read on the comments of kissmanga. It was said that in the novel, MC will forget the 100 million years after he finish it. which does make him use it again because he can't "remember" his...