1. There is a big difference between doing good because you sense that it is
somehow important to, and doing it because you were ordered to. However,
there is a psychological difference and a spiritual one; and while the first
can sometimes be a bit devious, the second can be highly empowering as we
will soon see.
Ramchal points out that our ability to act on our own free will (which, if
you will recall is the subject at hand) was first established at Mount Sinai
when we were granted the Torah, and when all the factors subject to free
will -- mitzvot and their opposite, sins -- were set into place .
However, as we alluded to above, our sages pressed the point that there is a
difference between "someone who is commanded to fulfill a commandment and
does, and one who is not but fulfills it anyway"(Avodah Zara 3a). On a
psychological level, one who fulfills it without having to is carefree about
it, feels good about himself for being so altruistic, and he is gladdened by
the fact that he’s his "own person". The person who is commanded to fulfill
it, on the other hand, feels pressure to do it and do it well and, the truth
be told, he sometimes feels pressed upon and "under the gun", since he may
not be personally motivated.
2. Something wholly otherwise comes into play here on a spiritual level,
though, as Ramchal reveals. For one who is indeed commanded to fulfill a
commandment and does is somehow granted an inchoate and mystical wherewithal
to better the world thanks to that mitzvah, whereas someone who simply
decides to do the very same act who's somehow "unqualified" to do it or has
not been appointed to from Above doesn't manage to accomplish much in the
process on a deep level (other than some possible harm, that is).
Consider the example of a Kohen who had been commanded to fulfill specific
and highly significant mitzvot in the Holy Temple. His having done that was
for the betterment of the world and it benefitted his own spiritual
standing. A non-Kohen who would step in on his own to do that very same act
would do terrible harm both to himself and to the world.
Even if a Kohen was in fact fulfilling a Kohen-specific mitzvah as he
should, but he was not wearing the necessary vestments, he too would be
doing harm (since the vestments were essential elements of the mitzvah, and
by not wearing them he'd be fulfilling a mitzvah that he -- since he wasn't
ready for it -- wasn't commanded to).
The point of the matter is that each element of the world and every aspect
of the mitzvot is an element of God’s will that something come about in the
universe.. And thanks to that our people have been granted the unfathomable
wherewithal to affect all sorts of changes in heaven and earth, to grow
closer to G-d in love and to cling onto His presence, and to bring on the
ultimate perfection through our free will.
 For Kabbalistic references see Klallim Rishonim 30; and R’
Goldblatt’s notes 1-2 in the text, and his note 82 on pp. 488-489 to his
Rabbi Yaakov Feldman has translated and commented upon "The Gates of Repentance", "The Path of the Just", and "The Duties of the Heart" (Jason
Aronson Publishers). His works are available in bookstores and in various
locations on the Web.