It's not that. I don't know, i don't like the NHibernate port for .NET. It's to much JAVA-alike for me. You have to do the mapping in xml etc... That's java practise not .NET!
There are also other nice frameworks out there, not only mine. For example XPO or others...
I don't get why you have to declare your relationships again in the xml files, they are already defined in the database!
Aren't there any frameworks that reflect your database schema and put them into businesslogic?
I took NHibernate because it's open source.
LLBLGEN looks very nice, but has it's cost also.