I don't know about "must not". Although it's "popular" among Christians and also people in the world to "brush under the rug" the possibility of anything negative happening, and it "feels better" to think everything is going to be "rosy", in reality, we still live in a fallen world, and with fallen people, and if they mean us harm, our "positive thoughts" can't "jump into them. Also, there's what I call being concerned enough about something to prod you take the necessary steps to thwart the potential problem {"Faith without works is dead."} "Denial" might make us feel better, but sometimes you have to be "concerned" enough to take action. Like, the other extreme of worry...example, if your have bad brakes and know it, the person not "worried" could be riding on the highway, going 70 mph, and if a tractor trailer stops short in front of him, you know what happens, WITH all those "positive thoughts", whereas the person who was "worried enough" to get their brakes fixed, is still alive. {Meaning, "constructive concern", if you don't like the word "worried."} So if the "worry prompts you to do something to strengthen your odds against the situation, like you use it as a protection, it has its place, And I would say, NOT worrying shouldn't mean riding on the highway with bad breaks, lying under a herd of charging elephants, etc. In other words, "going to the other extreme." I know that's not as "popular" an insight, but anyone who disagrees, I welcome them to ride on the highway with bad breaks. Be my guest.
