Because that is your family member and it is hard not to get hurt when someone who you love is yelling at you because you did something wrong. Its not very easy to get over something wrong that the one you love accused you of. Its a hard life like that.
Most people who criticize other's shortcoming in certain fields don't do it maliciously. Criticism isn't malicious. It's tough love. It's trying to help someone get rid of their negative aspects, whether it'd be laziness or not being good at math. Tough love is harsh, but in the long run it's good!
Why should I heed your position moreso just because you are shouting or pointing at me with your fingers? Your position on whether what I did was right or wrong and how I should act in the future is not logically more right or wrong just because you put a lot of emotion into it. I prefer to hear rational arguments and have little, no scratch that, NO respect for people who try to get me to agree to things by using emotional appeals. If you are truly right then I'll shrug off your attempt to use emotionality to appeal to me and agree with you since it's petty and pointless to object just because of emotionality when otherwise I would've agreed. But if I think I'm right and you think I'm wrong you'll have to convince me using reason, not emotion.