You have, I expect, heard Alexander Solzhenitsyn's widely-quoted line from The Gulag Archipelago:
... the line separating good and evil passes not through states, nor between classes, nor between political parties either, but right through every human heart, and through all human hearts.
The daily task of the Christian is to move his heart further and further into the good.
Now suppose the Christian places his heart next to another heart. Not above it, not below it, certainly not opposed to it, but next to it, with each heart aligned to that line separating good and evil.
What can the Christian do then but try to move the heart lying more on the evil side of the line toward the good side?
This is a simplified model, of course. In general, parts of both hearts may need to move for the line to take the same path through each. Zeal for the good is the fuel, though, regardless of which heart is being moved.
Once two hearts are aligned (more or less), they may move together toward perfection.