In Luke 15:1-10, the sinner who repents, causing joy in the presence of the angels of God, is not the sheep that was lost or the coin that was lost, but the shepherd who lost a sheep and the woman who lost a coin, but who, in each case, set about rectifying the matter (i.e. repenting).
In these parables, Jesus is the lost sheep, celebrated by the sinners searching him out; Jesus is the lost coin, with the sinners calling their friends and neighbours to celebrate.
Finding something (ultimately, in someone: Jesus) that we have an awareness of having lost, and the resulting joy when what was incomplete is made complete again, is at the heart of the Gospel-writers’ message (much more so than the forgiveness of sins, which Jesus only mentions twice; and one of those is to emphasis the action of a woman who already knows that she has experienced this).
As in Jesus’ day, in my own experience, those outside of the respectable community of the church are much better at this, than those inside.
Joy happens when the transcendent is made imminent. In our secular society, we have, within the church as much as outwith it, traded the possibility of joy for the imminent-made-transcendent, which is any form of escapism from the daily grind. The two are, at least on the surface, hard to tell apart; but only one results in an accumulative cure of chronic grumbling.
When did you last experience joy?