The answer is, "Chronic use of alcohol".
Maybe it could be that their brains aren't fully developed and they are going through a lot of different hormones which makes things more intense emotionally.
When Jarin drank a lot of water before going to the doctor, it is possible that his blood pressure will be low. It is because drinking water helps lower blood pressure for it helps eliminate unwanted elements in the body that contributes to increasing a person's blood pressure.
A printable Snellen Chart, which you can download online.
Something to cover one eye at a time, like an eye patch or a paper cup.
Scissors.
Tape or tack to hang the chart on your wall.
Pencil or pen to record your results.