Why the need to have such an overweening respect for someones religious beliefs?

As an agnostic I am curious as to why it is deemed disrespectful to question someone on why they believe what they believe when it comes to their religious affiliation. It is seen as healthy debate when people discuss and debate their political views and such like, but question someones religion and it then becomes disrespectful and personal attacks on my personal beliefs usually ensue. I find that just by questioning some peoples beliefs out of genuine curiosity as to why they believe what they believe normally results in a heated conversation. Why cant believers and non-believers discuss their views in an objective, open-minded way.