Jesus is offering a gift that can only come from Him.
37 On the last and greatest day of the festival, Jesus stood and said in a loud voice, “Let anyone who is thirsty come to me and drink. 38 Whoever believes in me, as Scripture has said, rivers of living water will flow from within them.” 39 By this he meant the Spirit, whom those who believed in him were later to receive. Up to that time the Spirit had not been given, since Jesus had not yet been glorified. - John 7:37-39
God made us for an intimate relationship with Himself, and we cannot get our deepest needs met anywhere else but in Him. Jesus offers the only satisfaction for spiritual thirst. In verse 38, from within them refers to the innermost recesses of the body.
Just as it was a miracle of God for water to flow from a rock in the Old Testament (giving life) it is greater miracle of God to have the Holy Spirit flowing from His people and eternal life.
Praise be to God for this gift!
Have a good Godly day.