"Joy does not simply happen to us. We have to choose joy and keep choosing it every day....You don't think your way into a new kind of living. You live your way into a new kind of thinking." (Henri Nouwen) 

God has gifted us with natural ability to think and to will. We can chose to think positively or negatively. Accordingly, our life will be directed. Just as we train our bodies through discipline, so we train our mind. This is what St. Paul meant when he said that we should take captive our thoughts. We may not be able to prevent what comes into our mind, but we can choose what we will do with these thoughts. A good see produces good fruit, whereas a bad see does not.

So, if I want to have a realistic but joyful outlook on life, then I must choose to do so and act accordingly. It doesn't happen overnight. But with practice this will become my frame of reference through which I view life. This is the joy that God wants for us.