They are synonyms, but selfless may, according to context, be a little stronger.
Here is a definition of 'selfish': devoted to or caring only for oneself; concerned primarily with one's own interests, benefits, welfare, etc., regardless of others
Add "un" to that as a prefix and it becomes clear the unselfish is not primarily concerned with one's own welfare. Selfless is essentially altruistic, concerned with others without regard to one's own self.
You may use unselfish and selfless interchangeably, but for my ear, selfless is just a little more emphatic.
I agree that "selfless" is a little to the right of "unselfish", or that it goes a bit further in behavior. Since both words are defined as meaning the same thing, why do we "feel" the subtle differences?
Maybe because the word "selfless" lends the feeling that the emotion causes you to shed more of your own basic needs in deference to the other person. So much so, that you become even "less" than your "self."
It starts tipping into imbalance in the personality, then, because it seems to go a bit too far. You feel yourself being pulled toward the next phase, which I would call "martyrdom."
"Unselfish" seems to mean a healthy way to live your life. Not too self-centered, just a positive approach in treating others with respect and sufficient attention.
But "selfless"? That word seems to come with a warning label, doesn't it? Do we instinctively know that no one should lose any sense of them "selves" - to the point where they become less than who they really are - all at the expense of someone else?
If you notice the "graph" you used, "selfish" and "unselfish" were balanced. When you added the adjective, "selfless", you placed it to the right of the graph, tipping it slightly and making it unbalanced.
Being less than "self" is something we don't want to be...not for anyone.
Selfless implies an extreme form of unselfishness. Being unselfish means that you are willing to share what you have, but not necessarily beyond your own means. Selfless means you would sacrifice everything you have and even beyond to help someone else. Someone who donates money to a charity might be unselfish, but Mother Teresa was selfless - giving with no thought to the possibly bad consequences for themselves.
Well possibly, but not always. In a holy context, as with Mother Theresa, the act of being selfless takes the meaning of that word to a higher realm. When used to describe saintly proportions, I agree with the definition of the word, "selfless."
But when we use it - and I believe we do sometimes- to describe a relationship between two people (as opposed to the heavenly, holy offering of oneself for God and fellow human beings), the danger for the negative extreme enters the picture.
To lose yourself in a partnership where you become a doormat, a weak version of who you really are, of being only what someone else wants you to be, that type of selflessness is negative and damaging. It diminishes someone and throws everything out of balance.
"Unselfish" seems to have a control button attached to it.
"Selfless" carries a warning over possible repercussions: "handle with care."
Maybe it's because I wouldn't want to be with a "selfless" person. Which is not to say I don't admire the purity and specialness of a Mother Theresa. In a one-on-one relationship, though? No, in that context, more balance produces better compatibility.