Paul transitions from the comfort to the church at the end of chapter 4, due to the rapture of 4:17 to the Day of God wrath in chapter 5. Chapter 5 is not a matter of comfort, but it is not something about which the church needs to fear, they are kept from it, because they are not subject to it.

This was written to the church in Thessalonica (primary recipients) but it is a matter of comfort to all who look for His receiving us to Himself. (secondary recipients) What happens when we are gone should help motivate us to be witnesses to people so they too may come to know Jesus, the one who has taken fear from those who believe.