"Joy is not the absence of trouble, but the presence of Christ." ~William Vander Hoven
Being joyful is a choice we must make, just as being thankful is. If we (Christians) wait for joy to find us in life, we will be sorely disappointed. God never promised His followers a life of ease, pleasure, smooth-sailing, and bliss; instead He tells us that we will suffer persecution, tribulation, hardship, and loss. True joy, however is not dependent on things or people, but on a genuine and growing relationship with Christ.
"Joy is not luxury or a mere accessory in the Christian life. It is the sign that we are really living in God's wonderful love, and that love satisfies us." ~Andrew Murray
Psalm 16:11 Thou wilt shew me the path of life: in Thy presence is fulness of joy; at Thy right hand are pleasures for evermore.
Philippians 4:4 says, "Rejoice in the Lord alway: and again I say rejoice." Rejoicing in the Lord alway doesn't leave much room for being unhappy. So while joy is a choice, it is also a command. Our obedience to this command is reflected by how well we live out Psalm 118:24 This is the day which the LORD hath made; we will rejoice and be glad in it. I wrote this verse down on a piece of paper at work for several days as a reminder to myself--I probably should still be doing that. :)
Proverbs 17:22 A merry heart doeth good like a medicine; but a broken spirit drieth the bones. Being happy really does help your health, just like bitterness really does destroy it.
Also, remember as I mentioned in a recent post, your joy (cheerfulness) will likely bless someone else. It's far easier to be happy when those around you are cheerful, and if they aren't you might just help them in that direction.
Joy is:
Jesus first
Other second, and
Yourself last
Love and joy ever keep company. Love, denying and forgetting itself for the brethren and the lost, living in them, finds the joy of God. The kingdom of God is joy in the Holy Ghost." ~Andrew Murray