How beneficial for Seneca and Lucilius to have such an intimate friend who both listens and coaches you to strive to be a better Stoic! Seneca's care seems to be very deep.
A true friend deeply cares and helps you; and it's stronger than love. A true friend learns how to truly love.
Friendship, accordingly, is always helpful, but love sometimes even does harm. Try to perfect yourself, if for no other reason, in order that you may learn how to love.
The ending part of the letter provides a bit more insight about how one makes progress. If you begin to feel a steadiness in your life and you are not tossed and turned at every emotion or change of circumstance, then you are making progress.
A shifting of the will indicates that the mind is at sea, heading in various directions, according to the course of the wind. But that which is settled and solid does not wander from its place. This is the blessed lot of the completely wise man, and also, to a certain extent, of him who is progressing and has made some headway.