While Jewish law obligates each person to observe all the mitzvot, there is always at least one particular mitzvah to which one's soul is drawn more strongly. According to Chassidic thought, this concept is hinted to by the fact that the Land was apportioned ''by means of a lottery'' [Pinchas 26:55] suggesting that one's primary mission in life, one's spiritual ''inheritance'' has been pre-allotted to him on High, and one has no choice in the matter.
The reason why a person's soul has a particular affinity to its ''special mitzvah'' is because each soul is a ''spark'' of the general soul which Adam possessed; and corresponding to its original ''location'' within Adam's soul, it will have a distinct spiritual path of its own. Consequently, through observing his ''special mitzvah'' the person will become spiritually fulfilled, which will give him additional enthusiasm in the observance of all the other mitzvot as well.
How can a person discover what ''his special mitzvah'' is? There is no simple solution. However, being that a person's mission in life is centered around his ''special mitzvah'', it follows that his ''evil inclination'' will oppose this mitzvah strongly, which may give a person a clue as to what it is. Furthermore, Divine Providence will inevitably lead a person to circumstances which are conducive to the observance of his ''special mitzvah''. [So if one is wealthy, for example, his special mitzvah is quite possibly to give charity.]
In addition to all the above, every person in a given generation will have a ''special mitzvah'' by virtue of the times in which he lives. And in our times, this is the obligation to promote knowledge, belief and yearning for the coming of Moshiach, since our souls have, by Divine Providence, been placed here in the last generation of Exile.
Source: Lubavitcher Rebbe - based on Likutei Sichos vol 2 p.346, Sefer Hama'amorim 5706 p.240, Sefer HaSichos 5752 vol 1 p.97