Whom having not seen, ye love; in whom, though now ye see him not, yet believing, ye rejoice with joy unspeakable and full of glory I Peter 1:8
Thou wilt shew me the path of life: in thy presence is fulness of joy; at thy right hand there are pleasures for evermore. Psalm 16:11
These things have I spoken unto you, that my joy might remain in you, and that your joy might be full. John 15:11
Happiness is dependent on circumstances; Joy is dependent on your Saviour.
I learned that little mantra in college. I have only just recently learned what it actually means. Before, for someone to tell me to 'have joy' was just about as effective as telling a five year old to 'have respect.' What you are really asking is for them to 'show' those things.
See, joy, respect, love are attitudes. Joy is one of the fruits of the Spirit listed in Galatians 5:22-23 "But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, meekness, temperance: against such there is no law." Those are all attitudes.
When something is an attitude that means it originates not in the mind, but in the heart. That usually isn't something you can change in a moment. To tell a child to show some respect does not change the heart, it changes the actions. It is a temporary fix. A show.
Some thing with joy. Joy is not something you can just 'show.' Happiness is what you show. Joy isn't walking around with a Miss America smile and laugh that drives people nuts. Joy is an attitude, a spirit.
Definition of Joy: something or someone that provides a source of happiness.
So, joy is the source of happiness, not its synonym.
How do we have joy?
We focus on Christ. We focus on the work He has done for us. When we understand how great our salvation is, when we understand how good He is, that we are His children, beloved, accepted into His family, when we understand all of that our hearts can't help but be filled with joy. It doesn't mean our outer man will always express joy. It does not mean that we will walk around with a plaster smile. What it does mean is that our hearts will be constantly filled with awe and wonder, with love, with joy unspeakable which will, at times, pour out onto our face.
"But none of these things move me, neither count I my life dear unto myself, so that I might finish my course with joy , and the ministry, which I have received of the Lord Jesus, to testify the gospel of the grace of God." Acts 20:24