As for why we need to go to confession before communing, St. Symeon says in his pre-communion prayers: "For Thou art Fire which burns the unworthy." Communion burns all that which is bad, and if we are not made pure by coming to confession first, we ourselves will be burned because of the evil which has not yet been confessed.
Please be careful in disseminating this point of view as dogma, though. We have already discussed the requirement of Confession before Communion in many other threads, and one could conclude rightly from our past discussions that your understanding is not the only doctrine embraced by the Orthodox regarding a link between these two sacraments.
To summarize, the practice of requiring Confession before EVERY reception of Holy Communion was originally a pastoral response to the trend over many centuries toward lay reception of Communion maybe once or twice per year. The return to weekly Communion in many Orthodox churches has made this requirement virtually unenforceable, and the subordination of Communion to the absolution received in Confession is a bad understanding of what Communion and Confession really are. I could go into much greater detail here, but, as I've already said above, many posters here already have on other threads. I'll just refer you to one of the most recent of these.Communion, Confession, Confusion