This freaked people out when gmail first opened years back
but the system is just automated
in fact as part of your privacy statement what you do is completely private
the us government about a year so so back actualy tried to make Google release search results to them to see what people were doing on the internet and where they were going in regards to certain things, most likely to intervene, but Google fought them to the end to protect the user rights
i'll say you have nothing to worry about
there is no person sitting there reading billions of emails saying oh this is about daycare...
just say 200 daycare ads, and if an email has a word of daycare in it they get placed there.
also on an irrelevant note... some search engines have no problem releasing information to the government... no names mentioned but if you feel paranoid do some research
hope this helped yoru paranoia !