It's a paradox, one that has to be experienced to understand: suffering leads to joy. Our Lord Himself gives us the example in embracing His passion and death. It all started in the Garden of Eden when our first parents sinned. They chose to be fooled by the Enemy into putting themselves before God, lifting themselves up in pride to refuse God's commandment and do as they pleased. The tragic result was original sin and its consequences: the flesh revolts against the spirit, and must therefore be kept in check with penance and suffering. St Paul will later speak of the "superbia carnis", the pride of the flesh erecting itself against God, thinking to find satisfaction in giving in to its impulses, but at the same time experiencing its weakness. How often have you not met people who say they are free to do what they want, but prove by their actions that they are enslaved to their passions? We read about them everyday....
That is why the Word Incarnate, our sweet Saviour Jesus Christ, chose the path of suffering. Suffering serves the purpose of curbing our lust for power and pleasure; it brings us down to the dust, which is where we should be, because that's what we are made of. Only humility saves, and suffering is a privileged path to humility.
This goes for physical suffering, but it also goes for emotional or psychological suffering. The greatest evil that can befall a human being is to have a too lofty opinion of themselves, for that is the sin of Satan, and it leads to revolt against the Creator, against the order of things which are as they are and must be accepted as such. We are in a universe we did not create, and our true grandeur lies in acknowledging that, in humbling ourselves enough to play our role in that whole, without usurping the rights of God, the greatest evil, one which our present-day world excels in.
That is why the cross is so important, why the Church celebrates this day with joy, for She knows that in spite of the difficulty we all experience in dealing with it, suffering is the divinely appointed way to being happy, in a very mysterious way, even in this life, provided we unite our sufferings with those of the God-Man and revolt not against Him. The Cross leads to Light, it leads to Glory. Let us beg for the grace to understand and embrace this all-important truth which moulds valiant souls, such that are pleasing to God and to men.