The null hypothesis signifies a lack of relatedness. Give circumstance A and outcome B, we make no assumptions, i.e. we assume A and B are unrelated by not assuming they are related. This is the null hypothesis. In order to reject that, we must provide compelling evidence.

If you start by assuming A and B are related, your experiment would have to fail reject that notion in order to prove that A and B are indeed related, which is a less direct way of approaching scientific discovery.