We are secure in the Lord because our loving, trustworthy, reliable, constant, gracious Savior and Lord is in control of all things. And He does all things well! So when He says, "In the world you shall have tribulation, but be of good cheer; I have overcome the world." John 16:33 We, who are trusting in Him, are to be just that, 'of good cheer'!
The Amplified Bible says,
"...be of good cheer [take courage; be confident, certain, undaunted]!"

Cheerfulness is a balm for all who take the word of Jesus to heart. Look at what Scripture has to say about a heart that is joyful and one that is not.
I'll bring in other translations to help open up some of the verses.
"A joyful heart makes a cheerful face..." Proverbs 15:13a NASB "A merry heart makes a cheerful countenance..." NKJV But, "...when the heart is sad, the spirit is broken." Proverbs 15:13b NASB "...by sorrow of the heart the spirit is broken." NKJV"

"A joyful heart is good medicine..." Proverbs 17:22a NASB "A merry heart does good like medicine..." NKJV "A happy heart is good medicine and a cheerful mind works healing."
Amplified

A joyful, merry, happy heart works healing. But what of one that is not like this?
"But a broken spirit dries up the bones." Proverbs 17:22b

Proverbs 15:15 in the Amplified starts with the negative first and is a dower outlook:
"All the days of the desponding and afflicted are made evil [by anxious thoughts and forebodings]..." 15:15a We can see here why Scripture tells us to, 'be anxious for nothing'
(Philippians 4:6) and 'rejoice always' (1 Thessalonians 5:16) The results then is, "...he who has a glad heart has a continual feast [regardless of circumstances]." 15:15b

"Anxiety in a man's heart weights it down, but an encouraging word makes it glad." Proverbs 12:25

Anxiety is not for us for we have been given, "...the oil of joy for mourning, the garment of praise for the spirit of heaviness..." Isaiah 61:3 And, "Cast your burden on the Lord [releasing the weight of it] and He will sustain you; He will never allow the [consistently] righteous to be moved (made to slip, fall, or fail). Psalm 55:22 Amplified

And again,
"Casting the whole of your care [all your anxieties, all your worries, all your concerns, once and for all] on Him, for He cares for you affectionately and cares about you watchfully." 1 Peter 5:7 Amplified

So with great joy we heartily hold on to the promise of Philippians 4:6-7, "Be anxious for nothing, but in everything by prayer and supplication with thanksgiving let your requests be made known to God. And the peace of God, which surpasses all comprehension, shall guard your hearts and your minds in Christ Jesus."

All praise and thanks to You, Lord God. Amen

NOTE:
NASB is the New American Standard Bible
NKJV is the New King James Version Bible All unlabeled verses are from the NASB