If you disobey God, says the text, there will be severe punishment. What kind of disobedience is referred to is both obvious and not obvious. That someone deserved punishment for not serving God – that was axiomatic for the Torah.
The less obvious sin is that a person may in fact have worshipped Him, though not “joyfully and gladly” (Deut. 28:47).
But how can the Torah command anyone to feel joy? An emotion is there, or it isn’t! Can it be summoned up at will? Since God knows the human heart, He will see that forced emotion is no emotion.
One way of approaching the question is by means of the concept of simchah shel mitzvah – joy in performing a commandment. If you are going to keep Shabbat, simchah shel mitzvah means: bring to it a positive attitude that says Shabbat is a great blessing and I am not going to start with a feeling of resentment that I have to deny myself that day all the things I do on working days.
The same goes with any mitzvah. If I carry it out perfunctorily, mechanically, simply because I have to, I will still have done the mitzvah, but neither I not the mitzvah will feel any satisfaction.
And one of the ways of bringing joy to the commandment is wherever possible to share its observance with others. Keep Shabbat not only for yourself but with others, and you both enjoy it more. (Some in fact feel uncomfortable if they have no guests with whom to share a Shabbat table.)
If you enjoy your Judaism, don’t keep this as your private secret: let others share the simchah that you feel.