I know what they say about how one should always think positively. People like to say they're "positive thinkers." But everything in creation has a purpose, even negative things. God created everything to benefit man. if you are always positive, you forfeit yourself of the benefits of negative thinking. So, when is negative thinking healthy?
Negative thinking allows us enough space to think things over. It's a factor in mature thinking. Inexperienced minds will just go on ahead and do it without realizing the possible effects later. I remember the guy who tried to fly his plane right into the eye of a terrible storm. He was so positive nothing bad would happen to him, until he realized too late that he was wrong. A little negativity there would have saved his life. It's useless to die like that.
I also remember the verse in Proverbs where it says, there's a way that seems right to a man. The man is positive that the way he is taking is the right one. He never entertains anything negative about it because he keeps hearing smart folks (especially in church) saying you should always be positive and negative thinking is foolish. Later, he discovers that the end of it is death.
We should learn how to apply a Godly balance of the positive and the negative in life. A good life is not all positive. You can never become strong to succeed against life's challenges if you never encounter the negative and prevail. God designed life--even eternal life--to have positives and negatives. Jesus had to face the negativity of the cross to have the positivity of the resurrection. Remember what he cried there? "Father, why have you forsaken me?" That was so negative.
Of course, no one could tell Jesus that he should not be negative about it. Not even the Father told him that. Why? Because the Father knew that negative things, faced with the right heart and mind, was good for his Son. The disciples were subjected to the negative effects of Jesus' dying on the cross probably to keep them meek and also to make all that lead to their maturity. Experience makes you wiser and more mature the next time around.
Remember to consider negative thoughts where they count because they may keep you safer and wiser in the end. Faith after all is not thoughtlessly dashing headlong to whatever it is that is before you. It is making sure that what you're doing is indeed God's will. That thought is quite relaxing, and a relaxed mind is a powerful mind.