Why does dropping of an OracleText index make other PL/SQL objects invalid

Hi,

I have one thing I have been  wondering about regarding ORacleText indexes. When I drop an OracleText index there are several packages and views (PL/SQL Objects) that gets invalid. It is just to recompile them again than everything works fine again (so no errors exists). But why is this happening and how can I understand/predict before dropping the index which PL/SQL objects will become invalid? Is there any dependency between an OracleText index and some PL/SQL objects? I could not see any dependencies when I looked into User_Dependencies before droping the OracleText index.

It would be great to understand why this happens.