I would like to know why the hair on the right side of my head is thinner than the hair on the left side of my head.

there are many causes for hair loss. To get the best treament option, you need to see a hair loss specialist and be examined from close up. 

Assuming you are a male and starting to undergo genetic hair loss, it is not unusual for one side of the head to thin faster than the other side. If it is genetic thinning, the other side will eventually catch up unless you treat it properly