The main problem is that ads are mainly hosted by tracking sites. If I look at a micro controller on a online shop, I see it over and over again while surfing

The main problem is that ads are mainly hosted by tracking sites. If I look at a micro controller on a online shop, I see it over and over again while surfing on completely different websites. These advertising provider do not only display some images or text which is including a link, they're also tracking and create a profile from me. And this is the main scary part of it: We don't have privacy anymore. Additionally it's nearly impossible to browse. People come to me and tell me their "computer is too slow, I think I catched a virus!". If you install an ad blocker, the computer works pretty well again. The next thing is that it's impossible to read a text when fancy animations are displays everywhere on the page. If advertising services violence agains privacy, make older computer und websites unusable, then don't be surprised that people do something against it. The same counts for the websites who are hosting the ads.