It seems to me the best explanation of arising of asava is following.
when this is, that is.
when this isn’t, that is not.
when avijja is, asava is;
when avijja is not, asava is not;
we know asava comes out in presence of strong arammana.
but we have asava because of ignorance of four noble truths AND tilakkhana.
we think this sankhata for which we have asava are delighting and worth having.
why? because, lack of understanding of their true nature.
our existence as lifestream is due to ignorance.
so, lack of panna is cause for sansara.
with panna, sansara is stopped for that lifestream.