“Kedoshim tehiyu… ki kadosh ani”. Chazal in a different context note the multiple possible meanings of the word “ki”, but the ambiguity of the term certainly stands out in the context of our pasuk. I think Seforno comes closest to what we would call the pshuto shel mikra (in the sense of plain meaning) in his interpretation: man must be holy because the telos of human achievement is living b’tzlem Elokim, in G-d’s image, and G-d is holy. Ramban in a similar vein reads “ki kadosh ani” as an assurance that one who lives a life of holiness will be connected to G-d because (“ki”) G-d is holy.
If “ki kadosh ani” is a reason, a justification, for the goal of holiness, I think we need to take a step back and consider the question that just that begs asking – why does being holy need a reason or justification? Why does there have to be a “ki kadosh ani” to inspire us to holiness? We certainly find in Chazal the principle of imitation dei, emulating G-d as a reason for doing virtuous acts, in the formulation of “mah hu… af atah” etc., but I cannot think offhand of a clear textual basis in Torah for such a formulation outside of our pasuk. Puk chazei mai wikipedia dabar in its entry on imitation dei – our pasuk of kedoshim and no other is quoted as a Torah source.
Perhaps the answer is that we don’t really subscribe to the principle of imitation dei as the be-all and end-all of virtue after all. The Torah commands “V’asisa hayashar v’hatov” not because G-d is yashar or tov (as these justifications are not stressed), but simply because being “yashar v’tov” are worthy ends in themselves. Virtue should be pursued not just as a means of imitating G-d or of connecting to G-d, but virtue is its own reward.
The same cannot be said of holiness. Without G-d as its focus, holiness has no meaning or virtue. Ramban writes that the opening mitzvos of the parsha parallel the 10 commandments, with “kedoshim” paralleling “Anochi Hashem Elokecha”, the mitzvah of belief. “Ki kadosh ani” is not just a tangential reason, but is an inherent and necessary ingredient of the mitzvah of kedusha. Whether you retreat to live as an ascetic hermit on some Tibetan mountain or take more subtle steps at trying to create artificial kedusha by injecting contrived spirituality into mitzvah performance, neither will succeed because by definition a holy life is a G-d centered life. Creating your own spirituality may warm your heart, but does nothing for the soul.