In Philippians 4:4-7, Paul instructs us to place our cares at God’s feet. To trust Him in all life’s circumstances and when we do we’ll find that His peace that passes all understanding will guard our hearts and minds in Christ 
Jesus. In this position we can truly find joy regardless of the circumstances that we might be going through. Only then can we do as Paul instructed the Thessalonians: “rejoice always, pray continually, and give thanks in all circumstances.” We will find a joy that transcends our problems as we entrust our lives into the hands of the one who gave them to us in the first place.
Paul then goes on the Philippians 4:8-9 and addresses us again and this time dealing with what we let ourselves think. Sadness, sorrow, depression and pain drive us to think such negative thoughts about life. It’s so easy to let the circumstances of our lives dictate how we think. When we allow our circumstances to drive our thought life, all joy will go out the window. This is why Paul talks to us in these verses about our thought life. He writes, “Finally…whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is commendable, if there is any excellence, if there is anything worthy of praise, think about these things.”
I like the way the Good News Bible translates Proverbs 4:23. It instructs us, “Be careful how you think; your life is shaped by your thoughts.” Discussing the various possible ways to translate this Hebrew phrase, the United Bible Societies Handbook for Translators says that in some languages this is expressed as “Watch your mind,” “Keep a hand on your head,” or “Take care of your thoughts.” We may also say, for example, “The most important thing you can do is be careful what you think” or “The most important … is to think good thoughts.” A website that describes AA uses of the phrase “stinking thinking” says, “Thoughts are a bit like nutrition and stinking thinking is similar to junk food. Those who live on a diet of junk food will end up paying for it. The same is true or stinking thinking.” Eugene Peterson translates Proverbs 15:13-14 in a similar way in that he contrasts joy that comes from focusing on truth and sadness & depression that feeds on trash. It says, “A cheerful heart brings a smile to your face; a sad heart makes it hard to get through the day. An intelligent person is always eager to take in more truth; fools feed on fast-food fads and fancies.” Or consider the New Living translation that says, “A wise person is hungry for knowledge, while the fool feeds on trash.”