Is there a reason why I should not cache nHibernate?

I just discovered the joy Cache.ReadWrite()in the free state of nHibernate and actively analyzed the results with nhprof.

This seems to be very useful, but it seems a little misleading. Is there any special reason why I would not want to cache a very often used object from a request? I mean, I have to assume that I should not just go around decorating each mapping with the Cache property ... or should I?