Why do parents focus on the negative aspect of teens?

Perhaps this is because parents hear awful stories from the media about teens. The media will capture any little rumor they hear and turn it into a big deal.
"Teen runs away"
"Teenage girl begins drinking"
etc., etc. etc.
Parents hear the news and worry that their teenage kids will become like that so they focus on all the things they are doing wrong.
Or maybe, parents just want the best for their kids and they want them to learn from their mistakes. 
Another reason could be that because many teens are going through puberty at that age, they begin caring about how they look and get worked up easily.
"Mo-om! I lost my hairbrush!"
"My curling iron isn't working!!!"
"My acne is just getting worse and worse! Ugh!"
Therefore, with their kids constantly complaining, parents are very likely to get frustrated about that. Either of those three reasons. I hope this helps!! Any questions, just ask me!
XD