The following explanation may help you to see why, at least in Kant's view, the Good Will is the only thing that could be considered good without limitation ( Samuel C. Rickless, 'From the Good Will to the Formula of Universal Law', Philosophy and Phenomenological Research, Vol. 68, No. 3 (May, 2004), pp. 554-577  :
[It] is stated at the very outset (at G 393): "It is impossible to think of anything at all in the world, or indeed
beyond it, that could be considered good without limitation except a good
will". Potter restates [this] as follows:
(Tl) A good will only [i.e., is the only thing that] has absolute worth.
What allows Potter to use the phrase "absolute worth" to characterize a good
will Kant himself describes as "good without limitation" at G 393 is the fact
that Kant uses (the German equivalent of) such terms interchangeably
throughout the Groundwork. At G 393-394, for instance, Kant characterizes
"moderation in affects and passions, self-control, and calm reflection" as having no "unconditional worth" (unbedingten Werth) and equates this both with
their failing to be "absolutely good" (schlechthin gut) and with their failing
to be "good without limitation" (ohne Einschrdnkung fur gut). He also
describes the good will as having "absolute worth" (absoluten Werthe) at G
394, strongly suggesting that he identifies the concepts of absolute goodness
and absolute worth.
The fundamental idea here, as revealed in Kant's own illustrations of Tl at
G393, is that anything other than the good will is conditionally good, in the
sense that its goodness is conditional (depends) upon the goodness of something else, whereas the good will is itself unconditionally good, in the sense
that its goodness is not conditional (does not depend) upon the goodness of
anything else. Kant then relies on the reader's ability to recognize that the
condition on which the goodness of something depends may reasonably be
described as "limiting" that goodness and as something to which that goodness is relative, and hence not absolute. And this is why Kant describes the
unconditionally good will as "good without limitation" and "absolutely
To probe a little further, note the following statements from the Groundwork :
A good will seems to constitute the indispensable condition even of the worthiness to be happy.
[The good will] must still be the highest good and the condition of every other, even of all
demands for happiness. (G 396)
[The concept of a good will] always takes first place in estimating the total worth of our
actions and constitutes the condition of all the rest. (G 397)
Kant is making two claims here. The first claim is this:
(1) A good will is the condition of every other good.
I take (1) to mean that every good thing other than a good will depends for its
goodness on its being associated with a good will. The second is a particular
instance of the first, namely that a person's worthiness to be happy depends
for its goodness on her having a good will. And this is not the only instance
of (1) to which Kant draws our attention. In the first two paragraphs of the
First Section, he tells us that talents of mind (such as understanding and
judgment), qualities of temperament (such as courage and moderation), and
other gifts of fortune (such as power, wealth, and health) are good only when
they are associated with a good will. This fact explains our finding the cool-
ness of a scoundrel "abominable" (G 394) and our taking "no delight in seeing the uninterrupted prosperity" of a bad- willed person (G 393).
None of this is new or surprising. But one of the more surprising facts
worth noting is that Tl follows from (1) (when conjoined with one implicit
assumption). The argument is as follows. (1) says that every good thing
other than a good will depends for its goodness on a good will. Assuming, as
seems reasonable, that there must be some good thing that does not depend
for its goodness on the goodness of anything else (for otherwise we would be
forced into circularity or infinite regress), it follows directly that this thing
must be a good will. (Were it anything other than a good will, (1) tells us
that it would depend for its goodness on something else.) Thus, a good will
is unconditionally good, and it is the only thing that is unconditionally good.
So (1), along with the claim that there must be some good thing that does
not depend for its goodness on the goodness of anything else, entails Tl.
I. Kant, Groundwork of the Metaphysics of Morals, tr. M. Gregor. Published by Cambridge University Press
ISBN 10: 0521626951 ISBN 13: 9780521626958
Samuel C. Rickless, 'From the Good Will to the Formula of Universal Law', Philosophy and Phenomenological Research, Vol. 68, No. 3 (May, 2004), pp. 554-577.
N. Potter, 1974. "The Argument of Kant's Groundwork, Chapter 1." Cana-
dian Journal of Philosophy, Supplementary Volume 1, Part 1. Reprinted
in Kant's Groundwork of the Metaphysics of Morals: Critical Essays,
ed. P. Guyer, 29-49. Lanham: Rowman & Littlefield, 1998