Rule #1 of being a YouTuber: Any activity on your channel is a Positive. YouTube does not gauge the quality of the activity. They see all activity as Engagement, whether Positive or Negative. Some channels thrive on negativity and are extremely successful.
As far as understanding the WHY of a dislike, don't worry about it. Like you, I used to want constructive feedback, because I wanted to learn and get better. Now, after four years, I have figured out that the old proverb is true. "You can please some of the people all of the time, and all the people some of the times, but you can never please all the people all of the time."
My channel has about a 10% dislike ratio. The majority of the reasons why people have told me they disliked my videos are pretty petty. Some people say my videos are too long. Others say they are too short. Some say I explained in too much detail. Others say I did not explain in enough detail. Some say I talk too much, so I replace talking with a silent montage with background music and then they complain they did not like my choice in music. Some say they don't like something I did because it was not the way they were taught to do it. And the list of petty reasons goes on and on.
If a person offers truly constructive advice, I try to learn from it. When people are just nasty, I simply respond by saying, "Thank you for commenting and promoting my channel." I have never had a single nasty angry person respond after I have said this line. It totally shuts them up. Either it confuses them and they don't know how to respond, or they realize that by attacking me, they are actually promoting me, and that is the last thing they want to do.
