It has been said that even when a person does a mitzvah with great joy, out of complete identification and delight, there should be an element of seriousness in the doing, a recognition that it is the Yoke of Heaven he is bearing, even when it seems light and desirable.
There is a certain difference between the Kingdom of Heaven and the Yoke of the Kingdom of Heaven.
It may be compared to the enraptured love of engaged couples and the mutual obligations and duties of married couples.
Love can remain magnificent and blissful throughout all the stages of a relation.
Why get married?
Why do we have to get mixed up with obligatory constraints and endless liabilities?
Indeed, it would be wonderful, perhaps, for love to remain free of all bonds, duties, and even promises.
But life seems to have decided otherwise, both in personal scope and in the national setting.
The day of the giving of the Torah at Mt. Sinai was the wedding day of the Jewish people.
–Rabbi Adin Steinsaltz
From The Candle of God, p.379, by Rabbi Adin Steinsaltz