When the מצורע (Mesora' - one who is afflicted with spiritual type of leprosy), wishes to cleanse himself, he has to wash his clothes, shave all his hair, bathe in water and then he becomes clean (see Wayyiqra 14:8). We can learn from this the manner in which a person cleanses himself of the sins he has committed.
When washing clothes that are soiled, the first wash takes off most of the mud and grime, however, some still remains. When washing a second time, the rest of the dirt comes off but there is a discoloration which can still be seen. Removing it requires a lot of scrubbing or bleaching.
So it is with purifying our sins. As soon as a person confesses his sins and truly regrets them, with the intention of abandoning them altogether, the major transgressions are discarded. However, the more subtle sins still remain. These ones require a much greater effort in order to cleanse oneself of them.
Additionally, the requirement to bathe in water, teaches us that just like water always seeks to descend to the lowest level, so too we must seek to be the most humble. In this way our repentance is accepted more easily.
(See Ben Ish Hai Derushim, Mesora')
Teshubah, 10 days of Repentance, Metzora, Mesora, Yom Kippur