In the Bible we are told that several disciples were going to Emmaus on that Sunday afternoon and while they were walking they encountered a stranger. They with this stranger for quite a while about Jesus and as suppertime neared they invited him to eat with them. They asked this stranger to say grace before the meal and THEN they knew that they had been talking to Jesus all along. This is the reason why we try to look at it as a joy-filled celebration, not just a solemn memorial. Even though we are sad because Jesus died for us, we are joy-filled because God raised him from the dead and he is alive forevermore.
Communion is a covenant.
Covenants are promises. Communion is about saying Yes' to God's promise to love us and care for us. When we say yes' what we really mean is that we will attempt to serve and follow God's way as best we can. When we take the communion elements and eat them, the love of God becomes a part of us, and we respond in love and service. Just like regular food gives us energy to live, communion gives us special, or spiritual energy.