Jesus just warned the apostles they'd be kicked out of their churches, killed by people thinking they're serving God, and then told them He's leaving to go to the Father...
Jesus says in verse 6 sorrow has filled their hearts and is preventing them from having joy that He is going to the Father. It was this sorrow that prevented them from focusing on their future, where Jesus was going. Jesus’s destination is where our hope is, and when Jesus told them what was coming, He was removing everything that could distract them from eternity.
This is one reason why they forsook Him, because they didn’t respond in faith to Jesus’ counsel. His counsel ends in faith that gives hope, and every purpose Jesus sets out to accomplish in our lives will always revolve around salvation through furthering the gospel. The good news isn’t that your church will accept you, or how much suffering you'll experience, or if they will agree with you. It’s about where Jesus went and what He did for us so we can get there.
Their heart was filled with sorrow because Jesus revealed their hope is actually hopeless. Hope in anything else outside of salvation is a guarantee your heart will fail when your temporal hope is destroyed.
This can create a problem in our walk with God when we expect God to honor misdirected hope. When that hope isn’t honored, you’ll blame God. Our solution is simple, if it ain’t eternal, give it a burial.
Our commission doesn't end at simply letting everything go, that’s where our calling begins! The next step is found in verse 5, “whither goest thou?” Where is Jesus? In the next passage of scripture v6-15 Jesus reveals to us that He's finishing His work through us! Making disciples, preaching salvation to the lost, healing the sick and setting the captives free.
All this is only possible by the Holy Spirit. Jesus says the truth will set us free, John 8:32, and it’s hard to set someone free from a lie you’re in agreement with. This is one of the tasks the Holy Spirit is set out to accomplish, leading us to all truth v13. That same truth Jesus revealed to the apostles that exposed the lie their hope was in, and if we have any misdirected hope, He will also reveal to us the end of that hope in the same way!
We need this deliverance because it’s the only thing hindering our obedience to Jesus, and failing to address it only leaves us ripe for a case against God after we forsake Him, hoping in a lie just like the apostles did when they forsook Jesus at the end.