The “what” of Easter is the resurrection of Jesus Christ. The “why” of Easter is so that the Spirit of God could live within humanity once more.
“Why” is a question that determines motivation. In other words, what should the motivation of our lives be as a result of the resurrection of Jesus? Christianity has become largely the celebration of what Jesus did, rather than a reception of the reason He did it. Jesus didn’t come solely to bring us to heaven. He came to bring heaven to earth. Jesus brought heaven to earth by bringing the Spirit of God back into the earth.
The reason Jesus brought the Spirit of God back to the earth was so that the Kingdom of God could be established on the earth. Therefore, the motivation of our lives should be to establish the Kingdom of God here on the earth and influence it with the culture of heaven.