Upadana paccaya bhavo (bhava means becoming - this is the thrill of getting what you want)
Becoming is an invisible process pushed into motion by our craving desire to repeat the thrill
worldly sensory thrills are transitory and fade, like the satisfaction from a coffee, or sex or a cigarette
Only when we see (insight meditation) that the striving after impermanent satisfactory sensations only leads to the 'repeat' effect and once the satisfaction fades, after a while, the craving for re-fulfillment recurs
this is what we call 'watta chakra' the turning wheel
We perpetuate the process, and it is this inclination that is present within our hearts, which is the driving energy of upadhana (addiction for a repeat experience)
This Upadhana sensation is precisely what is the cause of Bhava (becoming)
Now as to 'Becoming'
we never get to BE, only ever 'in the process of becoming'
we are all 'inbetweenies' and between leaving one state and becoming another
its all just a roiling sea of becoming and shapes trying to take on some substance and 'BE' (exist in a fixed way)
But they fall apart in the same moment these shapes rise up and try to form themself, and they fall back into the great ocean of impermanence
the closes we can ever get to really Being something is 'becoming'
We are always changing and in flux as is everything, because nothing can be held in the same state for more than a split instant (not even that because the unit of an instant vectors off into infinity too and has inbetween sub units of measurement forever.. there is no such thing as a 'single instant' so nothing ever exists even for an instant.. only is in process of becoming)