Kemonophonic said:
Why the fuck do people on YouTube keep putting their faces in their video thumbnails? It's so fucking off-putting and makes me not want to watch their videos.

Simple, humans are hard wired to recognize faces and therefore putting the face in a thumbnail attracts attention and with it more clicks an veiws.

There's also another reason which is it creates a weak, albeit present, human connection between the video and person due to the human face and therefore develop a minor emotional investment in the video's subject before they even see a second of it.