To make heart 100% pure is so difficult a task that even after infinite lives it is still incomplete.
We need help. Help comes from a genuine saint and without such help the dream of eternal bliss will never be fulfilled.

Saint when on earth offers many work areas where people can offer services to saint and earn spiritual advancement. Saint may appear to be in need of external help and if someone really does help, saint returns divine favor, which is his sole motive.
There is one such instance - On one occasion, Krishn's finger had some cut on it and it began bleeding. He rushed to Draupadi who could not bear that sight and tore a piece of cloth from her sari(clothing) and tied it on Krishn's figure. Bleeding stopped. Krishn returned her favor by proving unending cloth to cover her body when she was being stripped in public.

Thus one must understand whatever saint asks is for your spiritual advancement. The check - Saint must be genuine. Krishn at one place says,'Misdeeds done for me turn into deeds while deeds devoid of me are misdeeds.'
When a maid of Vrindavan went to meet Krishn on pretext of visiting temple Krishn welcomed it because she was on her way to 'Dismissing all religions, submit to me and only me...' To work for a genuine saint is same as to work for God. If you don't get such genuine saint, you can earnestly prey God for such meeting.

Saint not only works for purification of dirty heart but also turns the heart divine after purification and further fills it with the eternal and infinite bliss. Hence it is said that the Spiritual Master is more important than God.