... is one of the most offensive things you can say without resorting to obscenities.
Not "How about trying..." or "I would do this..." but Why? What is the reason? Are you stupid? Ignorant? Are you trying to mess it up? Is there something wrong with you? Are you afraid of something? Why?
Why don't you?
Never mind why you're doing it your way, never mind that, why aren't you doing it like this? Don't you know? Haven't you done this before? What is the reason for your flawed decision?
Why don't you just?
You're taking time over that are you? Why don't you just leave it as it is? Why don't you just do it my way? It's not even important, it doesn't merit a real solution, any actual effort, so just do this. Just forget it. It's just crap. You're wasting time. No-one cares. Just do it. Just get it done. Just leave it. What a fool.
Why don't you just get a job?
Why don't you just tidy up after yourself?
Why don't you just ask her?
Why don't you just get on with your work?
Why don't you just watch tv like everyone else?
Why don't you just do something worthwhile? Leave that crap, no-one cares. Writing your little programs - who's going to buy them? Why bother writing all that stuff, no-one's gonna read it. You think there's not better writers out there? Everything you think - someone's already thought it, written it, sold it, forgotten it. Why don't you just get on with your life, like everyone else?
Because, you fucker, just fucking because.