It’s a powerful sentiment. No question about it. It is a sentiment, however, not a fact. I think it works as well as it does because it sounds like a fact.
We are accustomed to statements about “worth” and this seems like one of them. We might say of alternative modes of transportation that the more expensive of the two costs more but saves time. “It is worth the extra cost,” we might say, estimating the value of the time to us by comparison with the value of the money. That is subjective, of course, because the value of your time is subjective, but both the cost and the time are external to you. You are the one who estimates their comparative worth.
That doesn’t work any more when you, yourself, are one of the items as well as the chooser.
Because the product that is associated with this slogan is a hair coloring product, we might make some progress by having the woman say, “…because having blonde hair is so important to me, I am willing to pay the extra money to get a good product.” That works the same way the transportation example worked. There is a chooser; there are two values external to the chooser and she is required to prefer one ordering to the other.
Even “It is worth it to me” works. But I think “I am worth it” does not work. What am I worth?
If the chooser is another person, someone external to the speaker, then we are back on familiar territory—personnel management, in this case. In sports, it is common to say that one person is more valuable than another. I would pay $10 million to acquire this player, but only $5 million to acquire that one.  That works just fine. The chooser is external to both the choices, even though both choices are persons, and the utility has a common metric.
What metric works for my judgment of my own worth?
I can think of one. Just one. There are some patterns of behavior that are widely recognized as belonging to relationships where worth is recognized or denied. These serve as templates. When I am given more than the template would normally allow, I am grateful and when I am given less, I am resentful. In both of those cases, we might say that the person is asserting a “worth” of himself/herself and making a judgment that the worth is recognized or denied or even exceeded.
This is a kind of half-example. There is a worth of the self, that the person himself/herself recognizes and that is accommodated (or not) by others. But this is a half-example only because of the use of the template. It is the template that is the external thing. It is used as an independent and external estimate about what a given person is worth. How should a supervisor be treated, a waitress, a talented artist whose work doesn’t sell, an ignorant and noxious professor who is beloved by his students? Use the template.
All this is background. None of it prepares us to take account of a woman who says she uses an expensive hair color because “she is worth it.” There is no external chooser here. There is no template of social expectations here. The woman’s worth can be measured not by how much the hair color cost her, but by whether her hair (herself) is worth spending that much money on.
You can see what is in this for L’Oreal. It attaches the cost of their product to the affirmation of self-worth by the woman who is buying it. Does she want to say she can’t afford it? Not really, because that slides downhill into not being really worth it. All the stress is taken off the cost, but it is not put, as it would ordinarily be, on the value of the effect. No, it’s just of the worth of the woman. The more money she is prepared to spend on hair color, the more she is worth in her own estimation.
That rationale got a little rich for some of the actresses who were called upon to look into the camera and proclaim their worth. Again, this outrageous proclamation doesn’t cost L’Oréal anything, but it apparently cost the actresses something. One story I heard said that it was Andie McDowell who finally said the line just didn’t feel right to her. She proposed, the way I heard it, changing the line to “…because you’re worth it.”
Of course, this changes the dynamic entirely. Now you don’t have to generate some (high) estimate of your worth. You don’t have to consult a social template of some sort to determine just how a person like yourself ought to be treated. All you have to do is to accept a few simple propositions from a beautiful and well-known actress. One is that she, who does not know you, knows what your worth is. Two is that the worth is very high. Three is that that worth justifies spending a lot of money on a product to change the color of your hair.
If you accept all three of those propositions, you go and buy L’Oréal and just hope someone chastises you for spending all that money on yourself.  If you want to bail out on any of the three, which will it be? Each is fraught.
Which is, I suppose, why it works. 
It does have that one little down side of equating your sense of your own value with the amount of money you are willing to pay for a product that advertises itself as unusually expensive. It is, bluntly, consumer spending as the indicator of self-worth that really costs us.
And I really don’t think it’s worth it.
 This is a little trickier than it sounds because of the way labor markets work. If a player is available who will provide $10 million of services to the team, you would pay $10 million to get him or her if you had to. But if there are other players available, you might not.
 The critic would not say “on yourself” or even “on your hair.” Possibly “on your appearance” or, if the critic is really nasty, “on cosmetics.” The critic knows that once “the self” is established as the receiver of value, the critic’s game is lost.
 Or why it “worked,” I should say. The current ads, I learned today, emphasize the health and strength of the hair, not the color. Health is good. I saw an ad recently promising “fiscal health.”
Fog Index 9.444