Answer: Wine symbolizes the Light of fulfillment (Hochma) that comes down to us during the holidays, thus letting us undergo a certain spiritual ascent.
Wine is not just a symbol of joy. Rather, it proves that we constantly enhance our connection and get closer to revelation of the upper spiritual world.
As soon as we reach a state called the crossing of the Red Sea and transcend from mutual detachment to reciprocal connection, we will happen to reveal new, benevolent relationships among us called the upper world.
There are four steps that we should go through in order to build a good connection among us. They are symbolized by the traditional four glasses of wine that we drink during the Pesach meals.
From Israeli Radio Program 103FM, 3/15/15