Our first reading tells us something about the relationship between God the Father and God the Son. It says that the Son is “the imprint of” the Father’s being.
We have something similar in Chapter One of the Letter to the Colossians where St. Paul wrote, “He is the image of the invisible God.”
St. Thomas Aquinas tried picturing how the Son could be “the imprint of the Father’s being,” and “the image of the invisible God.”
This is something like his explanation. As an intelligent being God had to be thinking. And, since before he created anything, the only object he could picture was himself. And, being unlimited in his mental powers, he had a mental picture of himself that was a complete copy of himself. It was his brainchild. And, since love is the necessary attraction to what is good, the Father loved his brainchild.
The next step in Aquinas’s theorizing took the mutual love of Father and Son as never changing. It became a person in its own right.
Of course my account of Aquinas’s great reasoning is pretty weak, but we all must do our best at understanding the Scriptures.