Well - this was an interesting thread over on the Puritanboard and should give folks a good idea that even though we are Calvinistic, we still "work out our own salvation" and the practical application of our doctrine. It's not always pretty...or simple...or "sweet baby cakes" loving, but it is the truth.
Bottom line - how can we trust that God is not cruel? Why doesn't He save everyone?
You can trust God, because you can trust Christ.
Christ displayed the character of God in that He showed an abundance of love toward His children, even asking forgiveness for those that crucified Him, yet was ruthless toward those that dishonored the Father.
If you don't trust Christ, you are a son of the devil and deserve the fate of the enemy of God.
Mercy's preciousness is measured by it's rarity, otherwise there is no true justice.
That is why not ALL are saved.
Remember, cause 1 - Man's rebellion - everyone WAS saved - WE screwed it up - and if you think you would have done differently, you are FOOLING yourself and should examine yourself for sinning in pride. If God does not punish His enemies and the bearer of His image for rebellion, He is a liar and there is no true justice.
If He does not display mercy, there is no true love.
Remember, God is the only one capable of true justice, not Man. We are, however, capable of expressing love, imperfect though it may be, thus our mandate in the second part of the greatest commandment.
We will rejoice at God's righteous and just punishment of the reprobate and rejoice that we have been saved from among them, not because of our worthiness, but because of God's mercy.
Soli Deo Gloria!