it appears that our "sense of lack" is the drivinf force, our motivation to acquire more and more. acquisition, here, is not merely the acquisition of "things", but is (one might say) "infinitely extended". extending the concept acquire to an "infinite degree" requires that one includes, within the realm of "things" to acquire, all thoughts, beliefs, hopes, people, tools, items, etc..
the "sense of lack" becomes manifest in manifold ways. it manifests as desires, perhaps most subtley. most basically, though, it appears to be characteristic of a "moving mind". when the mind is propelled (or compelled), for any "reason", into movement; that is, the moving of itself, towards some desired end, it is said to be acting from the "sense of lack".
this may all be understood, by you, and you may really be concerned with why there is a "sense of lack" to begin with, so i will attempt to enlighten this discussion as to what the source is, of the sense, or feeling.
who is not whole, is incomplete. from this incompleteness, arises the "sense of lack". but, then, how is it that one is not "whole"? this "problem" arises from a "wrong notion". it arises, precisely, from wrong identification. what is it, then, that one is wrongfully identified with? one is identified with something that is not wholly real, namely, a self that is inherently meaningful and seperate, individual. we think that we are independent entities; meaningful in our own, but Buddha tells us, "No. there is no self as you think there is. that self that you are identified with is no more than a thought; as substantial as a fleeting thought. like a thought, which has meaning only in relation to other thoughts, you are empty of inherent meaning. identification provides the "sense of self". you are what you identify with. when that identification is erroneous (that is: not in accord with The Real, Reality, Truth) the "sense of lack" arises as a natural course of causality."
now, the historical Buddha never said this, exactly. but, then, the historical Buddha is not Buddha. that is: he is not the only one to be referred to as Buddha. Buddha is this mind. Buddha is the actualization, by realization, of the Original Mind. All are Buddha, but how few Know It! How few know Buddha!
Buddha is, when "self" is not. Buddha is concealed by the "idea of self", in other words. when there is emptiness, there is Buddha. Buddha immediately fills what is empty. if one is already "full" of theories, ideas of self, judgments, prejudices, etc. then Buddha is concealed and is, therefore, conceieved of to be "not-present".
"sense of lack" is the feeling that we all have until we realize Buddha; our Buddha-hood. every kind of identification one chooses or obtains for themself, is equally a "thing" that destroys emptiness; negates Buddha-hood. when one is utterly empty, then one is completely full and no longer has any "sense of lack"; one is no longer "anything", it immediately comes to pass that there is the realization that there is only Buddha! and That is everything! in emptiness, it is utterly full; filled to the brim.
note: any idea of self is incomplete and therefore erroneous. self is only complete, when there is no imposition of an idea of it. as a result, it is exactly what it is, in reality. it has fulfilled and satisfied its nature, by being what it is and negating all that it is not.
one cannot think to act naturally. one cannot produce a state of naturalness. one cannot force naturalness, through any conceivable idea. naturalness come from the negation of all that runs counter to naturalness. all conceived ideas of self, are invented and applied to a process, extraneously, that is already functions naturally; in accord with Nature. to identify with an idea of self is over-kill. one already is what one is, so there is no need to conceive of it otherwise.
"sense of lack" is a cyclical phenomenon. we feel lacking because we think we need to be something more or different that what is. we do this because we feel a "sense of lack".
all is understood with the realization of Buddha-nature, and the cycle is transcended. this cycle is also called "samsara", "the wheel of birth and death", "the cycle of re-birth". (re-birth refers to the idea that we are constantly being reborn in a new idea and dying to a former one. this never ends, until one realizes Buddha-nature.)
hope you have understood, and that i have provided sufficient clarity.
Buddha is great. indeed!
"if mind moves itself, then unmoving mind is self" - Jonathan B. Bostick