A merry heart maketh a cheerful countenance: but by sorrow of the heart the spirit is broken. (Proverbs 15:13)
All the days of the afflicted are evil: but he that is of a merry heart hath a continual feast. (Proverbs 15:15)
A merry heart doeth good like a medicine: but a broken spirit drieth the bones. (Proverbs 17:22)
"Merry" is defined as:
Gay and noisy; jovial; exhilarated to laughter
Causing laughter or mirth; as a merry jest
Brisk; as a merry gale
Pleasant; agreeable; delightful
Now, I understand that everything in this life isn't "merry." I understand there are a lot of things about this life that are difficult. I recognize that many things you and I face simply aren't any fun. But I would venture to say that none of us have it as bad as the early Christians to whom Peter wrote his first epistle. Read verses 3-6 of chapter 1:
Blessed be the God and Father of our Lord Jesus Christ, which according to his abundant mercy hath begotten us again unto a lively hope by the resurrection of Jesus Christ from the dead, To an inheritance incorruptible, and undefiled, and that fadeth not away, reserved in heaven for you, Who are kept by the power of God through faith unto salvation ready to be revealed in the last time, Wherein ye greatly rejoice, though now for a season, if need be, ye are in heaviness through manifold temptations.
These believers were suffering greatly for their faith in Jesus Christ. They were in heaviness. They were experiencing "manifold" temptations. Yet they had cause to rejoice. Why??
They were recipients of God's abundant mercy, verse 3.
They were born again, verse 3.
They had lively hope of a resurrection, verse 3.
An incorruptible, undefiled, unfading inheritance was reserved for them in heaven, verse 4.
They had God's promise that He'd bring them to that place, verse 5.
So no matter what is happening in your life right now, if you are a child of God, you have all of that and MUCH MORE. So praise God! Lighten up. Enjoy yourself. "Don't worry – be happy." Take your cares to Jesus, and rejoice IN THE LORD always (Philippians 4:4-7).
Look back at the verses in Proverbs and consider the results of a merry heart. A cheerful countenance, as opposed to a broken spirit. A continual feast, as opposed to days that are evil. A good medicine, as opposed to dry bones. The condition of your heart affects your whole being – spirit, soul, and body. If you can figure out how to keep your mind on God and His blessings and be "merry," it'll improve your fellowship with and service to God, it'll improve your fellowship with and enjoyment of your brothers and sisters in Christ, and it'll even benefit your physical body.
Some practical thoughts/suggestions:
It is obvious that God has a sense of humor. Be godly.
The Bible plainly states that Jesus Christ was anointed with the oil of gladness above his fellows (Hebrews 1). Be Christ-like.
Thus, if we are to be "godly," we're going to have to give this merry heart thing a shot.
If Christianity is what makes you always seem ill or angry, why would anybody want it?
Learn to laugh at yourself. We're all dust, and none of us are perfect.
Courtesy chuckles are even good. When somebody tells a bad joke, consider it an opportunity to practice your fake laugh.