We can enjoy the sunshine but cannot control the weather . . . There are many times we can't control our income. A plant downsizes, heating bills jump drastically, an unexpected illness drains your reserve. Some things are beyond our control. But Paul does seem to imply that the attitude of rejoicing is something that we can control.
Happiness is tied to circumstances.
We are happy when things go well, when we get a promotion, when we hear the applause of the crowd, when we know success. We do not enjoy those times. Joy is different from happiness. Joy is an attitude. It is an approach to life rather than a reaction to life.