I really do "get" being joyful as a Christian, and all the good things of Christ and what He's done for us, contained in our salvation due to His sacrifice on our behalf. But, I also am, like He was while on this earth, a "man of sorrows." He looked around at all the grief and agony that was happening, and felt sad for the people and what they suffered, resulting from their own sins, from the sins of others, or from circumstances beyond their control. And so He was sad; forlorn. And that's what I am feeling today, from so many things that still are bringing pain, even though I know that He has already restored all things in the eternal realm, and that He has, and is, creating a new Heaven and a new Earth, whose Kingdom will have no end.
But, in the meantime, we go through times and experience these sad feelings as part of "the fellowship of His sufferings," in this way. We are feeling what He felt as He looked around at the ravages of sin and the enemy on people's lives. If He didn't feel these things, He would be cold and heartless, and we know that is not our Agape Lord and Savior. Our Oneness with Him is not only the joyful experience of the flow of His life-giving Spirit in us, but also the humanity of His soul, until "all enemies are put under His feet," and manifested on the natural plane. (Although, in the spirit realm, they have already been defeated.)
It's as if our soul is crying, "how long, Lord-how long, until our faith becomes sight-until joy comes in the final morning?" The Day of the Lord will bring great sorrow for those who don't know Him, but release and an entirely new existence for those who do, which we are ever moving forward in, even today. Until then, the weights of this world bare on our souls.