There are often questions about relationship between members within the Trinity. John 14 is usually at the front of the list. I think James White is helpful here in his explanation.
“You heard that I said to you, `I go away, and I will come to you.’ If you loved Me, you would have rejoiced because I go to the Father, for the Father is greater than I” (John 14:28).
Probably no passage comes to the lips of the person who denies the deity of Christ faster than John 14:28. Yet if we will but consider the passage, and avoid embracing surface-level uses of it, we will find that it does not lead us to deny the deity of Christ, but rather to embrace it.
Most of the time we see this passage only partially quoted. The last few words are recited as if they by themselves settled all question of the deity of Christ. “The Father is greater than I.” Doesn’t that say it all? No one is greater than God; therefore, Jesus can’t possibly be God if, in fact, there is anyone greater than Him. How could it get any simpler than that?
But such an argument ignores what Jesus himself is saying.
Why does He refer to the Father as being greater than He is? He does so because He is reproaching the disciples for their selfishness. He had told them that He was going back to the presence of the Father. If they truly loved Him (and were not simply thinking about themselves), this announcement would have caused them to rejoice. Why? Because the Father is greater than the Son.
Now immediately we can see what the term “greater” means. If it meant “better” as in “a higher type of being,” these words would have no meaning. Why would the disciples rejoice because Jesus was going to see a being who is greater than He? Why would that cause rejoicing? But the term does not refer to “better” but “greater” as in positionally greater. The Son was returning back to the place He had with the Father before the world was (John 17:5, see below). He would no longer be walking the dusty roads of Galilee, surrounded by sin and sickness and misery. He would no longer be the subject of attack and ridicule by legions of scribes and Pharisees. Instead, He would be at the right hand of the Father in heaven itself.
So we see that the term “greater” speaks to the position of the Father in heaven over against the position of the Son on earth. The Son had voluntarily (Philippians 2:6) laid aside His divine prerogatives and humbled himself by entering into human flesh. He would soon be leaving this humbled position and returning to His position of glory. If the disciples had been thinking of the ramifications of Jesus’ words, they would have rejoiced that He was going to such a place. Instead, they were focused upon themselves and their own needs, not upon the glorification of their Lord. –James White, The Forgotten Trinity.