Likewise you also should be glad and rejoice with me. (ESV)
We are programmed to respond to people who are hurting. Our first response is one of pity. In fact, if someone says, "My loved one just passed away we respond with, "I'm sorry." I am not trying to minimize any kind of loss or negative circumstance but I want you to think for a moment about the way in which we respond to others when we hear that their situation is less than perfect. Pity.
The Apostle Paul is writing this letter from a Roman prison. He has to rely on the kindness and support of others or he will rot to death in his cell. The Roman Empire was not going to provide three square meals for the Apostle. He was severely beaten and imprisoned when he was in Philippi. And his life has been marked by all kinds of distress. So, does Paul expect pity from the Philippians?
Paul says, "You also should be glad and rejoice with me." That becomes a more powerful statement when you consider the one saying it. Paul understood the difference between joy and happiness. Happiness is a temporal feeling based on circumstances. As you know happiness is a fleeting feeling. Joy is a deeper decision to find contentment regardless of the circumstances. Paul had joy.
Perhaps when someone tells us of their negative circumstances we should react a bit differently than we are normally inclined. We can certainly grieve with them. It is not wrong to say, "I am sorry you are hurting." Perhaps we can add to that, "Have you found the Lord to be your strength in the midst of this time? How can I pray with you?"
Joy or happiness? The world tries to offer happiness, but it is fleeting. Joy is for the believer. As you draw closer to Christ you can find joy, no matter the circumstances. Chasing happiness? What a pity.