Why are there controversies in the Gemara i.e. why did it start?

There are various causes for controversy in the Gemara, which was written after there was no longer a Sanhedrin, so there was no vote taken as to what is the clear decision. In short, among the causes: different ways understandings or deducing from the Mishna; how to resolve between apparently conflicting mishnayot or braitot; different ways of learning from the verses in the Torah; different ways of applying the accepted rules of halacha when conflicting; different ways of thinking; over time- new questions arose and the amoraim differed as to how to apply previous precedents; over time- some traditions were forgotten; originally there were sometimes several legitimate ways of doing things, but eventually the rabbis preferred to have 1 way, but they differed as to which way.