Mako was in shock by the commander's words as there wasn't only wisdom behind them but genuine pain and guilt as well.
He could feel the commander's emotion through his emotional voice and trembling hand which showed him that he wasn't the only one going through this kind of thing and that all leaders since the very beginning of mankind had to deal with this exact same feeling.
"Son... always remember... In a battle, there will always come a time when a leader has to make a difficult decision, a decision that could ultimately result in the success or failure of everything. In those times, remember that you are only one person and there is only so much you can influence." The commander told Mako in a calming voice.