Basically, the fat man is saying that parental love has to come second to one's duty to the country.
What he says is that parents do not have children for their own (the parents') sake. They have children for the sake of the children. When your sons get to a certain age, they want (among other things) to serve their country.
At that point, he says, you have to let them go. You should be proud and happy that they want to serve the country. If they die, you should be happy because they died doing what they wanted to do. They died happy, so you should be happy too.
So he is saying that parental love is too selfish. Parents need to think of the kids and the country, not themselves.