Before knowing the reason why ASMR videos can relax you, you need to know what is ASMR first.
ASMR is a deeply relaxing and comforting feeling, usually accompanied by tingling sensations in the head. The"tingling" starts in the back of your neck, travels across your scalp, and moves downward, following the line of your spine. It might be explained by cognitive neuroscience, and that kind of relaxing feeling truly exists and I think people can benefit from it.
You can experience ASMR getting a haircut, being treated by a doctor, watching someone paint skillfully, hearing someone whisper, or listening to crinkling sounds — the triggers are endless and vary from person to person. The reason why ASMR videos can relax you is that those ASMR hosts could meet one of those aforementioned triggers that do function to you.
As for me, after watching lots of ASMR videos, I prefer to watch some chewing sound ASMR videos, due to my addiction to delicious food. When I watch someone have fried chicken with relish, it will trigger my ASMR which makes me feel relaxed. Below is one of those ASMR videos about chewing sound recorded by Scenes Lifelike VR earphones. If you are also a foodie, I think you will love it.