Since according to sikhi, the ultimate aim of human life is to become ONE with god, I wonder why souls separated from god in the first place?
P.S. I don't wanna hurt anyone's feelings so please bear with it if my question sounds arrogant.
Thank you for asking this foundational question; it is this curiosity that helps many of us understand the essence of Gurbani without getting bogged down by incomprehensible philosophical arguments.
The answer to your question " why souls separated from god in the first place" is:
BECAUSE WE CAME INTO EXISTENCE BY
DEFAULT.
Let me please expand.
When a candle is lit - there is light around by default.
When sun shines, there is sunshine by default.
In Jap ji Sahib we daily read ( and it is well acknowledged by science about Big Bang) :
ਕੀਤਾ ਪਸਾਉ ਏਕੋ ਕਵਾਉ ॥
ਤਿਸ ਤੇ ਹੋਏ ਲਖ ਦਰੀਆਉ ॥
When God decided to transform Himself ( ਪਰਗਾਸ) from the state of nothingness (Nirgun State) to Sargunn Saroop and the creation came into being, we all were born by default as Jot of that supreme Jyoti - as the light of the source. We came into existence
by default because God decided to transform Himself. Thus we are created by default the moment God decided to step out of the Nirgunn state as "Jyoti" - we came into existence as His "jot" ( His light).
Unfortunately given the immense beauty of His creation ( for which Gurbani uses words like Vismaad), we as "jot" started to identify ourselves more with the visible creation rather than with the invisible source behind. The source is invisible because it has no shape or form(ਨਾ ਰੂਪ ਨਾ ਰੇਖ ਨਾ ਰੰਗ). Even if there is some shape of form of this invisible source ( God/Jyoti), it is beyond human intellect to fathom and describe.
Since we have started to identify ourselves with the Creation, we developed this blindness ( a veil of separation) - a duality. Gurbani is teaching us how to transcend this restrictive barrier so that the jot can get unentangled from Maya ( Creation) and get absorbed back into the source when the intensity of light finally fades in us.
Why would the intensity of light fade in us? The more distance we create from the source; the intensity is likely to diminish. Distance is not how far we are from the source in mathematical terms; it means how intensely we are entangled with material infatuations and thus adding thickness to the veil of separation - i.e. strengthening our ego.