It doesn't matter what you believe, as long as you're happy and not hurting someone. Can't we all just get along?Put like that, I too have an instinctive desire to simply agree and shut up. Yet, I can't, and here's why: belief matters. It absolutely, positively matters what you believe. Sometimes it's of great consequence, and sometimes it's of very little consequence. But it matters.
Belief matters because it has direct, and sometimes indirect, effects on our actions. It can't help but do this, or put another way, we can't help but act according to our beliefs. If you believe that apple seeds contain enough cyanide to kill, you probably will make sure you don't swallow them when eating an apple. If you believe that ginkgo biloba can enhance your memory, you're more likely to spend money on it.
Those are small beliefs; it's even easier to see how large beliefs affect us. If you believe that "what goes around, comes around," or some version of karma, you're more likely to let certain offenses pass, since karma will bite them in the ass. On the other hand, if you believe that there's no such thing as karma, you're more likely to see a value in confronting the offense, rather than let it pass. If you believe that there's an afterlife, then you're more likely to be willing to risk your life, since death is not the end. But of course, if you believe there is nothing like an afterlife, then you're less likely to take that risk.
Our beliefs can also affect our happiness and general satisfaction. If you believe that everything works out for the best, always, you might be less likely to give in to depression when the going gets rough. Of course, you might also be less likely to work as hard as you need to get past that rough patch, since things "always work out." It's not always clear how one's beliefs will affect your emotions or actions.
But affect them they will. When I believed in reincarnation, karma, and a God that loved me, I made a conscious decision to consider ethical dilemmas as if all that wasn't true. Mostly, I think I succeeded. Yet, that decision was itself influenced by the belief that I had to share a world with people who didn't agree with me, and that there was a possibility, however small, that I was wrong.
This is why I care what you believe. Your beliefs affect what you do, and what you do might affect me. When it doesn't, that's when I quit caring -- but only until the next belief is confronted. I am happy to get along with you, with anyone, so long as your belief doesn't cause harm, or at least, is very unlikely to cause predictable harm, and so long as it doesn't interfere with the rights of others. But I will not say it doesn't matter what you believe.
After all, I'm sure it matters to you.