If someone is sometimes happy and sometimes not happy, it means that they are still situated on the material plane being tossed this way and that by the modes of material nature because on the spiritual plane there is nothing but uninterrupted, ever-increasing happiness.
I wake up enlivened every day knowing that Lord Sri Krishna has blessed me with another day to increase my Krishna consciousness and the Krishna consciousness of others.
The law of karma gives out good and bad reactions precisely according to our previous good and bad activities from this life and previous lives. Therefore if bad things happen to good people, it means that they are not as good as they appear to be. They had some bad karma lurking in their karmic closet, and now it has finally come out and caught up with them. In a similar way, a bad person may have some previous good karma from past pious activities that he is now enjoying and consequently exhausting.