WHY MOURN ON A GOOD FRIDAY?

For those who regret that Jesus died:

From that time Jesus began to show to His disciples that He must go to Jerusalem, and suffer many things from the elders and chief priests and scribes, and be killed, and be raised the third day.
Then Peter took Him aside and began to rebuke Him, saying, “Far be it from You, Lord; this shall not happen to You!”
But He turned and said to Peter, “Get behind Me, Satan! You are an offense to Me, for you are not mindful of the things of God, but the things of men.”
--Matthew 16:21‭-‬23

Jesus rebuked Peter because it was Jesus' plan to suffer, die and to rise on the third day but Peter was talking against it.

For those who weep for Jesus died:

Even there were people who were mourning when Jesus was carrying the cross. Read it below as it is written:

And a great multitude of the people followed Him, and women who also mourned and lamented Him.
But Jesus, turning to them, said, “Daughters of Jerusalem, do not weep for Me, but weep for yourselves and for your children. For indeed the days are coming in which they will say, ‘Blessed are the barren, wombs that never bore, and breasts which never nursed!’
--Luke 23:27‭-‬29

Jesus' death was the gift that He brought for us to this world.

Because...

"... God demonstrates His own love toward us, in that while we were still sinners, Christ died for us." --Romans 5:8

Don't treat Jesus the way you treat a man. He is the God who came to pay the price for our sins by dying in our place. He redeemed us with His blood. Make a joyful unto Him.