First answer - that's the million dollar question

2nd - Probably not the answer you want - but a cople factors are present. First things first - it's Google - so a LOT of things mean a little

1 - what's your sitemap state?  Your sitemap can tell google how often you want it to return. A sitemap will help google know what pages are there -but that doesn't mean google will index all of it.  But it's a step in the overall picture

2 - how old is your site - has it been around for a while?  Google seems to not like newer sites.  Longevity of a site speaks volumes (along with activity)

3 - Do you have good content? Content is king with google - and the fact that your competing with (portentially) millions of other sites -  you must have a LOT of it -and it's all got to be relavant.

I could go on - but that would be a whole SEO course - and there's plenty of info on the web.