There is a great deal of bridal or wedding spirituality in the Old Testament: that is, relating to God as to ones spouse. To take just one example, in the Book of the Prophet Isaiah, God says: You shall be called `My delight is in her. For the Lord delights in you. As a bridegroom rejoices over the bride, so shall your God rejoice over you.
There are very good reasons why a Christian disciple might fast. But they are just reasons. They are not a gloomy and miserly attitude that can appear very religious while being nothing of the sort. A reason may be valid for a particular occasion or a particular time. But the same reason may be unreasonable for another occasion and time. Joy is the chief characteristic of a Christian, joy even in tribulation. Joy is one of the first fruits of the Spirit.
In todays Gospel, Jesus is referring to such passages when he replied, How can the wedding guests fast while the bridegroom is with them? While the spirit of joy is around, be filled with it. Then it will stay around us a little longer. And when we have to move into sadness there will still be some effect, some coloring of that joy in us. Our sadness will not be miserable but quiet and deep. But if our joy is measured and miserable, our sadness will be an even greater misery. Our joy and our sorrow compensate each other.