If you are part of the YouTube Partnership Program (YPP) or considering applying for it, it is important to follow the YouTube Monetization Policies - Community Guidelines, Terms of Service, Copyright, and Google AdSense program policies. Failure to do so may lead to difficulties with monetizing in the future. When YouTube reviews your content to see whether or not guidelines are being followed, they typically look at:
Main theme of channel
Most viewed videos
Newest videos
Biggest proportion of watch time
Video metadata (i.e. titles, thumbnails, and descriptions)
Full details around Monetization Policies and how they are enforced, please see YouTube's Help Center.