In Thessalonians 5: 18 it says "give thanks in all circumstances; for this is the will of God in Christ Jesus for you."
I've been thinking a lot about this verse lately and especially in the times when I'm finding it hard to be thankful. Before, when reading this verse, I always thought the '"for this is the will of God" part was referring to the fact that His will was for us to be thankful. I still think that is his will, but even more so, I think the "for this is the will of God" is referring to "all circumstances". All of our situations (happy, sad, difficult, fun) are God's will for our life... and that is why we should give thanks in each one of them. In each and every situation God is forming us to be more like him. God allows suffering and hard times in our lives (whether caused by external factors or our own personal sin) to teach us, mold us, humble us, and remind us of our need for His grace.
Suffering is a blessing, and that is why we should give thanks! God not only allows suffering to change and teach us, but also to grow His Kingdom. I'm currently reading the book "Instruments in the Redeemer's Hands" and the author Paul David Tripp, when talking about using our suffering to witness to others, says this:
"God has brought me through sin and suffering, not only to change me but to enable me to minister to others. My story is a small chapter in the grand story of redemption, and Christ is on center stage. My story is much more about him than it is about me. In this way even my failures result in his glory. In my own weakness, foolishness, and inability, I have learned the truthfulness of his promises and the reality of his presence. This makes my story a vehicle of change in the lives of others."
So, we can be 'thankfully suffering' because our suffering has grand and eternal purposes, for our hearts and even the hearts of others.