I think a lot of people do wonder, especially when they are well but sometimes in depression when they doubt themselves. However, think about it this way. If anything causes emotional distress to the point of limiting your ability to function - some kind of mental illness is likely to be at play. Now what kind of mental illness is up to the doctors but it isn't normal to not be able to function regardless. If you were functioning just fine with no decreased abilities or effects in your personal life whatsoever, then I would be questioning whether mental health was a factor. When I see medication helping me, that is all the evidence I need to keep taking it. Your medication is for bipolar, it wouldn't help if you didn't have it. If you've ever missed a dose by accident and felt the wrath, that's what you need to focus on.