Why is using synchronized in setter bad?

Question

Java

This question came up in an interview.

Answer 1

Perhaps because the setter must be used to set the value of a single field, and using synchronized for this is redundant.

Answer 2

Because it usually doesn't make any sense.
Unless you take into account something like

void setVar(int var) {
  if(var == null) {
    this.var = var;
  }

but it is no longer a setter.
And in the case of a simple assignment, there is no difference between two consecutive value settings in two consecutive calls or in two parallel ones.