For awhile, I was getting way too many DNF books. Then I changed my sources of book recommendations and now I only get a few a year. Find a Goodreads group that reflects your interests, look up authors you like at
www.literature-map.com. If you like scifi and fantasy, go over to worldswithoutend.com and look up some award winners. Look at what mobilereads.com readers are reading - there are several threads of this sort. If you find a member who likes the same books you do, look at the other books that they read.
OK, so after that advice, why do I quit? Usually it turns out to be one of three things. It is YA (I have enjoyed very few of these), way too much graphic violence, or first 1/3 of the book is too slow. I'm tolerant of a lot of writing styles and even some bad writing if the story is good, but, if you don't keep my attention, I'm not going to slog through it. I also avoid books that have a complex plot or a ton of characters because I have difficulty keeping them straight. But this is an issue due to brain injury rather than a fault of the author.