How Can We Always Rejoice? (Philippians 4:4-9)

When we face challenges in life it is easy to complain about challenges.  I doubt that the first thing we think in our minds is that we just rejoice in the midst trials.  On the one hand rejoice in trials might just make a mockery of the common curse of this age.  On the other hand it might just dismiss the seriousness and the struggle of the trial.  So, how can Paul command us to rejoice in the midst of trials?  How can we rejoice always without being superficial regarding life’s challenges?