Well common sense also says the earth is flat and the sun revolves around it, that doesn't mean it's right.
Really, you ought to take at least the modern portfolio theory part of a finance class and if you haven't already a stats class before that. Like many things in life, this is non-obvious and "common sense" will give you the wrong answer. Anyone wouldn't say no, in fact anyone who had a basic finance education would ask a few more questions about correlation before they offered an opinion of any kind.