Why did God become His own Son?

He didn't. You are misunderstanding the way the scriptures were written at that time. The Hebrews then are much like the Arabs and hispanics of today, very sentimental using lots of imagery and parables in talking. They love to tell stories. They called each other brother and to every man, God was the father. Every person is and was seen as God's son. But it was the later "Christians" who deliberately misinterpreted these scriptures to put forth their own ideas.