"How can I write back to my parents" means "Why should I write back to my parents". "Zhi" is omitted here.
Being a man is not only to enjoy life, but also to repay kindness, to make progress, to comfort the kindness cultivated by parents and to serve the society. In this way, it is the person you should be, the luck of your family and the happiness of society.
Extended data:
Don't stop paying because you don't have the best return, and don't stop starting because you don't have a perfect result.
Lose in hesitation and win in action. Life without pain and fatigue is boring.
Everyone is desperate, so redouble your efforts. You have no reason to complain about the coldness of the world at the same time. The world won't stop because of your weakness. If you are not brave, no one will be strong for you.
Everyone else is trying. What qualifications do you have to be lazy? When you envy others' success, why not look at others' efforts? It is better to surpass them than to admire them.