Thomas Brooks emphasizes the contrast between earthly joy, which is often interrupted by sin, Satan, and afflictions, and the perfect joy that awaits believers in heaven. He describes how, in this life, joy is mixed with sorrow and challenges, but in heaven, believers will experience pure and everlasting joy without any disturbances. Brooks highlights that the joys of heaven are beyond human comprehension, filled with fullness and eternal pleasures at God's right hand. He encourages believers to look forward to the ultimate joy that will encompass every aspect of their being in the presence of God.