Why do testicles drop?

The best temperature for growing sperm is a little COOLER than body temperature. That's why the scrotum and testicles hang (unprotected!) outside the body. So if you wear tight pants and lots of warm trousers you won't have as many sperm as a Scot who wears nothing under his kilt.
The testicles drop into the scrotum because the body is too warm to make sperm optimally. The scrotum allows the body to regulate the temperature of the testicles a few degrees below that of the body temperature.
In Puberty and beyond :

They drop away from the body to be at the right temperature. The sperm needs to be at the correct temperature to stay healthy.

The best temperature for growing sperm is a little COOLER than body temperature. That's why the scrotum and testicles hang (unprotected!) outside the body. So if you wear tight pants and warm trousers you won't have as many sperm as a Scotsman who wears nothing under his kilt.

The testicles drop into the scrotum because the body is too warm to make sperm optimally. The scrotum allows the body to regulate the temperature of the testicles a few degrees below that of the body temperature.

In Babies :

They descend into the scrotum shortly after birth (Balls drop) not at Puberty as many believe. If they have not visibly dropped into the scrotum by several months after birth then a visit to the doctor is advisable to make sure all is ok with the childs health.