How do spanish speakers change names to show affection?

If there name is a common name like Juan they change it to John but if not there parents or they will pick another random name

It depends. Some changes depend on their characteristics, skinny, fat, etc. We usually add "ito" or "ita" at the end of the name.
Ex:
Pancho = Panchito (Male)
Cristana = Cristianita or Cristy (Female)

Us Mexican's normally use "wey" to friends.
Ex:
Olle! Que ondas wey! = Hey! How's it going man/ girl?

It honestly just all depends our relationship with the person, just like in the English language. If you don't like the person, why give them a nickname? If you love/ like the person, of course we're gonna give them a nickname.

Hope this helps!~

Este as como estes de tinen