Answer:
First, remember that mathematics is a logical construct.
So, when something has no logical sense, it is undefined.
Now, let's think about a division by zero:
If you have $100, and you want to divide it between 0 people, how much gets each one of them?.
Naturally, there is no logical answer to this question, we can not know how much gets each one of these people because we have no people to divide that money.
Now, we can find another logical problem if we assume that the division is actually defined.
For example if 1 divided by zero is equal to a number A, we have:
1/0 = A
ok, if now multiply both sides by zero we get:
(1/0)*0 = A*0
1*(0/0) = A*0
1 = 0
So, 1/0 can be defined only if 1 = 0, again, a logical problem.
This is why the division by zero is not defined.