Gorillas are shy Most of them never fight.
How would you combine this sentence using a conjunction. Would it be but or so?
If you write: "Gorillas are shy, so most of them never fight", it makes sense. They're shy, therefore they don't fight.
If you write: "Gorillas are shy, but most of them never fight", it implies that being shy would make them more likely to fight. This doesn't make any sense, so you know that the conjunction "but" does not fit well in the sentence.
Therefore, the proper conjunction to be used in this case would be "so".