We live our lives for God because of what he did for us at some point in the past, as if that point is the greatest moment of the Christian life.
Salvation isn't a moment, it's a work that Christ began in us in the past and will complete someday in the future. (Philippians 1:6)
We shouldn't live our lives in light of one thing that happened to us in the past. We should live because of a relationship that will continue 'til the day of Jesus Christ.
Because, that's what Christianity is, a relationship. It's not a contract that we signed in order to get heaven in return for a life of "doing right." It's a relationship that God started with us.
The best day of a marriage shouldn't be the wedding. It's a big deal, and a day to remember, but it shouldn't be the best day of the marriage.
It's the same with our relationship with God. We should remember the first day, but the first day shouldn't be the best day.