Simply the body's natural requirement for liquids ?
When he tries to choose
Sankhara comes up
and make a list of what he likes and dislikes.
Then, if he chooses based on like or dislike
he is on Upadaana level.
Karma intensified according to the amount of Sankharas.
Bhava becomes a little or much longer.
If he uses pragna to choose
with upeksha
being equanimous,
Bhava does not happen,
and since he is aware of the Sankharas
they too get eradicated.
Vaya dhamma sankhara,
Uppada vaya dhammino
uppajjithva nirujjhanti.