Thanks!I'll reply as a viewer. Usually I watch the whole video if:
1. the video is right about what I expected and wanted to watch (and my expectations are based on the title and thumbnail you made);
2. the video is enjoyable to watch (good narration, good personality of the creator, good enough video and audio quality, etc.);
3. the video is valuable for me (either it is educational or entertaining value).
Make interesting video with good quality of production, that's the answer for me.