A rich man called Jesus as "Good Teacher". In reply Jesus told him "Why do you call me good?" further Jesus added, "No one is good but God alone". What does Jesus mean here. Is Jesus not good? Why does Jesus separate himself from God. If God is good, Jesus also must be good.
The word "good" in Luke 18:19 is Strong's Gr. 18, "ἀγαθός" or "agathos", and it means "describes what originates from God and is empowered by Him".
In the corollary of Mark 10:18 Thayer's Greek Lexicon has the definition under #2, "what is upright, honorable, and acceptable to God:" (1)
The context is the key to this question. In this situation, Jesus was confronted by a man who was puffing himself up, and Jesus headed him off at the pass. Jesus directed the man's attention to the Father who is God over all, the Most High, the Great I AM. As Jesus was obedient in all things, He is the reflection of the Father, and His messenger.
For this same reason, when John questioned Jesus about immersing Him, Jesus told John it was for righteousness.
"15 And Jesus answering said unto him, Suffer it to be so now: for thus it becometh us to fulfil all righteousness. Then he suffered him.
16 And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him:
17 And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased." (Matt. 3:15-17, KJV)
This incident with the wealthy man occurred during Christ's ministry, before His death on the cross. Jesus still had to meet all righteousness, and be obedient in all things to fulfill all things (John 19:28, Acts 3:18) to be able to offer that righteousness that we might all partake of.
So, for righteousness sake, for obedience in all things, Jesus would not allow the man to believe another "man" to be good. Psalms 14:1, 3; 53:1, 3; Micah 7:2 all lament that there is none that are good. Confirmed again in Rom. 3:10:
"10 As it is written, There is none righteous, no, not one:" (KJV)
After providing the answer, the man protests in vs. 21,
"And he said, All these have I kept from my youth up. (KJV)
His answer showed that he believed himself to be good, but Jesus countered and told him the one thing that he still did not have - faith enough to follow Christ, to believe. The man was putting his faith in his wealth and possessions, instead of in God, and the message that God's Son brought.
We may think that we can be good enough on our own, but that is false. People may say, "I don't steal; I don't commit murder; I don't lie; I don't commit adultery," etc. But it is not our goodness that saves us, because we have all sinned in one way or another and are all under condemnation / judgment.
There is only one definition of good that counts with God, and that is through His Son's obedience and righteousness. We can never be good enough on our own. The wealthy man thought he had done everything he needed to do. But he was not willing to give all of himself to Christ.
"...and come, take up the cross, and follow me." (Mark 10:21, KJV)
Jesus was not separating Himself from God. He was affirming that all righteousness is in God the Father, and that the Son was obedient to the Father, and doing the Father's will in all things.
Note 1) Strong's Gr. 18 - Biblehub