"親になった今、父親がよく機嫌が悪かった理由がわかる。"

Simplified Meaning:

When a person becomes a parent, they start to see the world differently. Suddenly, they have a lot of responsibilities like taking care of the baby, going to work, paying bills, and dealing with everyday problems. This can be very stressful and tiring, which can make a parent feel grumpy or upset sometimes. For example, imagine a parent who works all day, then comes home to cook dinner, help with homework, and clean the house. They are probably very tired and might get annoyed easily. Understanding this, we can see why parents sometimes seem in a bad mood—they are simply overwhelmed and exhausted. Knowing this, if you see your parent acting grumpy, you might understand they are just dealing with a lot of pressure, not because they are angry with you. By keeping this in mind, we can be more patient and supportive of our parents, maybe even offer to help out more at home to lighten their load.