Jesus was sanctified because:

Jesus prayed:

Sanctify them by Your truth. Your word is truth. As You sent me into the world, I also have sent them into the world. And for their sakes I sanctify myself, that they also may be sanctified by the truth. -- John 17:17-19 (NKJV)

This is significant because "sanctify" also sometimes translated as "consecrate" in the Greek context the original word hagiazó could mean:

• to make holy (purify)
• to treat as holy (dedicate)
• to set apart as holy (separate)
• to consecrate (complete holiness process)

For example:

Husbands, love your wives, just as Christ also loved the church and gave himself for her, that he might sanctify and cleanse her with the washing of water by the word, that he might present her to himself a glorious church, not having spot or wrinkle or any such thing, but that she should be holy and without blemish. -- Ephesians 5:25-27 (NKJV)

Note that this is a verb which means it is an action to make the person holy and not an existing attribute of the person.

We were not born holy, but we could be sanctified (separated for dedication and purification for God's purpose).

However, if Jesus was God, and we apply this same word to him, then:

• God had to purify Himself (from what?); or
• God had dedicated Himself (to whom?); or
• God had to separate Himself (from what?); or
• God had to complete Himself (from being incompletely holy?)

None of these statement makes any sense for God.

However, Jesus did baptize himself with the baptism of repentance (Mark 1:4; Acts 13:24; 19:4):

Then Jesus came from Galilee to the Jordan to John, to be baptized by him.

John would have prevented him, saying, “I need to be baptized by you, and do you come to me?”
But Jesus answered him, “Let it be so now, for thus it is fitting for us to fulfill all righteousness.”

Then he consented. And when Jesus was baptized, immediately he went up from the water, and behold, the heavens were opened to him.

-- Matthew 3:13-16 (ESV)

If Jesus was without sin (1 Peter 2:22; Hebrews 4:15; 1 John 3:5) and the purpose of baptism is an act of repentance from sin, then Jesus' baptism would be redundant. It even surprised John the Baptist, when Jesus showed up to be baptized.

Therefore, Jesus explained that baptism is necessary to "fulfill all righteousness".

But if righteousness mean:

to be acceptable or in right standing with God

... then the view that "Jesus is God" fails, because it does not make sense that Jesus had to get baptised by John, a human prophet, to be accepted or at least be obedient to himself.

But if Jesus was a human, then his baptism and sanctification make sense because it dedicated Jesus for God's special purpose and fulfilled his righteousness with God (as Jesus said it would) so that God publicly announce Himself:

"This is My beloved Son with whom I am well pleased." -- Matthew 3:17; Luke 3:21