Why are mathematical proofs that rely on computers controversial? There are many theorems in mathematics that have been proved with the assistance of computers, take the famous four color theorem for example. Such proofs are often controversial among some mathematicians. Why is it so?
I my opinion, shifting from manual proofs to computer-assisted proofs is a giant leap forward for mathematics. Other fields of science rely on it heavily. Physics experiments are simulated in computers. Chemical reactions are simulated in supercomputers. Even evolution can be simulated in an advanced enough computer. All of this can help us understand these phenomena better. 
But why are mathematicians so reluctant?

Here's a great article on why.
There are several reasons. One of the biggest ones is that how one solves the problem is frequently more interesting and useful than the actual result. If that how is "we did an exhaustive search of all the possibilities," you don't really get much other than the result. There's also the risk that relying on computers removes the need to improvise with novel ideas.
Another reason is that bugs and faults in the computing are sometimes considered too much of a risk to mathematical certainty.
