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Why did Arius argue that the Son had a beginning?

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Christology The Transfiguration, the nature of Jesus as the Son of God being revealed to the apostles Peter, James, and John, mosaic icon, early 13th century; in the Louvre, Paris.

Arius argued that the Son had a beginning because he maintained a strict commitment to the uniqueness and absolute unity of God. In his view, God is the only being who is self-existent—meaning not dependent on anything else for existence—and immutable (unchanging).12

Because Arius believed the Godhead is unique and cannot be shared or communicated, he concluded that the Son could not be the self-existent God. He reasoned that if the Son were mutable (capable of change) and not self-existent, he must be a creature brought into existence by the will of the Father. Consequently, Arius viewed the Son as the first of all creatures, created out of nothing to serve as the instrument through which God made all other things. This perspective allowed Arius to uphold a strict monotheism while interpreting the term "Son" in a way he felt conformed to general usage.123