@isekaimagus
"But why a veterans of two years want to leave the military to become an adventurer?"
Two years won't make necessarily make you a veteran, and unlikely to shoot you high into ranks. To become a veteran, you'd need to actually fight in wars. 2 years could be dedicated to training, maintenance and drills.
If you want in-world explanation for example, it could be because adventuring is more lucrative than being a soldier. Basically, a clever medieval fantasy lord could pull a massive hat trick here.
First you spread rumors about adventuring being lucrative, then you add requirement to serve two years in army, then you add a loop hole, where anyone who ever served in the army could be drafted in case of war. As a result, you get influx of recruits, when any of your greedy neighbors try to borrow pieces of your territory, all those recruits will be available, and the adventurers will serve as extra peace-keeping force. Obviously, this is not information from the manga, just an explanation of HOW it could work, HOW it could be organized, and HOW it could make sense.
Regarding the plot, they're going with classic scenario where MC meets someone who leaves deep impression and want to become just like them.
Additionally, MC is archetypical shounen "saint-kun". Meaning strong convictions, strong opinions, at the same time extremely naive, and as we go further into the plot, there will be probably dramatic screaming and dramatic lecturing of a villain. And the like.
This is an interesting plot device, but the author will probably fail to use it in a clever manner.