Remember,you are also making a decision to give the tracking companies, of which Google is the largest, but there are an many more, your information that helps them build a dossier that is sold to the marketers, and of course is available to hackers, governments, etc. And that personal information, like everything else Google collects, becomes their property and is retained forever in the profile they have about you. The reason the ad is "relevant" to you is that Google also knows where you live, drive, your searches, the content of your emails sent and received, every picture you've taken and where, who your friends are, the items you order, the music you listen to, etc. As an intelligence official once said, "If Google didn't exist, we'd have to invent it."
OK, I'll bite...
Here's an excerpt from an article on Android Central that explains how Google handles your info and selling it:
They know what we search for online, what we buy from Amazon (and other places), where we have been and places we've investigated and even how we got to the places we've been to. That's some scary stuff, but we need to remember that Google disassociates it all from your personal identity as it's collected and processed. No human being is reading your stuff, because there is too much stuff to read. Things about you are marked down and your unique advertising ID is how they keep track of what they think is important. And you have some control over all of this. Visit yourGoogle My Account pages and see just what you're sharing, and how you can manage it all. Opting out of interest-based ads is easy, though it doesn't mean Google stops collecting the data — they just stop using it in this way.
Google can then tell me that if I pay them X amount of dollars for advertising, they will be able to show my ads to devices (your phone, your tablet and your computer) being used by folks whose advertising ID shows they are interested in going fishing. My ads will also show in a rotation for people who have opted out and don't get interest-based ads, but the bulk of exposure my product will be getting is targeted to the people my research shows are most likely to buy my product or people who are searching for things that makes a computer think they like to go fishing.
If Google sold any of this information to anyone else, they wouldn't be able to offer this unique service to people wanting to by ad space. The amount of data Google collects can be scary, and the ways they collect and process it all is a bit confusing and technical (robots!), but they aren't selling your data. It's too valuable to let it go."
Long story short, the stance that Google "sells your data" and "lends it to the government" is a myth perpetuated by Apple. No advertiser buying from Google ever knows who you are (you're just a number) and it would be asinine to think that Google wouldn't try to keep your most private info secure--it's what the company is built around. Failure to do so by them would be a death wish.