A common objection from many varieties of the faithful from many religions.
“What business is it of yours if I believe a magical space fairy farted the universe out after eating the primal beans?”
Here’s a little roleplaying exercise to help you understand.
Imagine I come up to you and say:
“I believe that by cutting out your heart with this obsidian knife and offering it to the gods I can ensure a bountiful harvest.”
I submit to you that, at this point, it becomes REALLY FUCKING IMPORTANT what you believe. Especially as it relates to me.
Maybe you don’t want to actually kill anyone, but keep in mind that your irrational beliefs can and will have an impact beyond yourself. Think of the position that gay people find themselves in with regard to marriage rights. Consider the abortion rights of women. Consider the dangerously ignorant teaching of creationism in schools.
People with irrational beliefs vote, protest, act, agitate. They try to enforce their own beliefs over others who do not share them – and not for the common good.
Quite apart from anything else, the perpetuation of the idea that faith is a valid reason for… well… anything is dangerous in and of itself. Belief without evidence is completely nuts and once someone has abdicated reason in such a way is hard to dislodge from it because, having rejected evidence they’re unlikely to accept it in refutation what they believe or to show something else as true.
I care what you believe because it directly and indirectly harms us both.