In 1 Peter 1:15-19 the Apostle Peter reminds his readers that they are to be holy because the one who called them is holy (vv. 15-16). Peter then predicates this command to be holy upon what Christ did for them on the cross. He says that they were purchased, not with perishable things such as silver or gold (v. 18), but with the precious blood of Christ (v. 19). Our motivation for doing good, therefore, should not be the mere command of God.
Simply being told to be holy will not give us the ability to achieve holiness. We need a deeper motivation than a simple command. First, we need to have a work done within us. We must have the precious blood of Christ applied to our lives. Then, second, we must remember the sacrifice of Christ as we pursue holiness. Only when we allow our affections to be stirred for Christ will we have the motivation to put aside our sinful desires and live holy lives. Our sinful desires connote just be restrained, they must be replaced with greater desires. Those greater affections must be brought about by deep meditation on the bloody cross of our Lord Jesus Christ.