Robin died when he was just ten due to his asthma.
After that, he was reincarnated into an unkown world as the crown prince's twin brother of some empire where he was allowed to live a comfortable life.
However, as everyone kept comparing him with his brother and even there were gossips about how much of a burden he was to him, Robin made up his mind to run away from the empire and give up his position as the second prince.
A carefree life as a commoner wouldn't be that bad after all, everyone would be happy.
It's easier to think it than doing it though.