How about a completely over-the-top example:
You're allergic to peanuts - the kind of allergic that'll kill you if you so much as look at one.
I love peanut M&Ms.
You can flat out ban me from eating M&Ms or you can explain to me that for your own safety you are unable to be in a relationship with someone who eats peanuts. In the first case I might think you're just being unreasonable and preventing me from eating my favourite snack for no good reason. When you sensibly dump me after finding a half-eaten bag of M&Ms under my pillow, I get to tell my friends about how much of a control freak you were and how you dumped me over chocolate.
In the latter case I'll probably not only avoid M&Ms, I'll also read the back of the packet before I eat other things just to be sure. If, on the other hand, I'm a bit of a dick and decide to eat peanut M&Ms anyway, I'm doing it in the full knowledge that our relationship is now over and I'm the one who killed it.