"And we know that for those who love God all things work together for good, for those who are called according to his purpose." Romans 8:28

"...for those who love God, all things work together for good. The meaning is that God causes all things to work together for good; for "things"--that is, circumstances--do not work together for good themselves. Rather, God directs the outcome of those circumstances for our good. The "good," however, is defined in verse 29 as our being conformed to the image of God's Son. In other words, Paul is telling us that God intends all our circumstances, both good and bad (But in the context Paul has in mind, especially the bad ones), to be instruments of sanctification, of growing us more and more into the likeness of Jesus.

So in situations that do not turn out the way we hoped, we are to give God thanks that He will use the situation in some way to develope our Christian character. We don't need to speculate as to how He might use it, for His ways are often mysterious and beyond our understanding. ...

...the giving of thanks in a disappointing or difficult situation is always to be done by faith in the promises of God. It is not a matter of doing it by sheer willpower. If we do that, we are giving thanks with our lips but not with our hearts. But as we cling to the promises of God, we can say, "Father, the circumstance I am in now is difficult and painful. I would not have chosen it, but You in Your love and wisdom chose it for me. You intend it for my good, and so by faith I thank You for the good You are going to do in my life through it. Help me to genuinely believe this and be able to thank You from my heart."-Jerry Bridges, (From: Respectable Sins)

Here's another verse from the Bible which exhorts us to be thankful when we are anxious and upset: "..do not be anxious about anything, but in everything by prayer and supplication with thanksgiving let your requests be made known to God. " Phil. 4:6

It's very difficult to say, "Thank You Lord for this cancer." Or to say, "Thank You that I lost my job." But if we understand this is happening for our good, (those of us who love the Lord), because the Word tells us so, then the pain, though it may be crushing us, will be buffered by the peace of God, which guards and rules our hearts, through His grace and love.