>>75455
Well, Berine does it because he's a soldier and that's his duty. He's doing it for something as simple as patriotism. Io Fleming's reasons are more nebulous, but it's easy enough to deduce that the destruction of his colony and his father's suicide motivate him to fight against Zeon.
Both of them have certainly more reasons to go fighting a war than a teenager that suffers a mental breakdown any time he has to man up and actually kill someone.