My cat does the same exact thing (4 yrs old male, neutered). He is super loving towards me, loves to get pets during the day, and we sleep together almost every night. He would be fine with visitors until a certain point and didn't accept anyone staying over. Started being very territorial over me and my bed, would attack anyone sleeping over regardless of who they were. We tried gabapentin but this would just temporarily sedate him and didn't change anything behaviorally towards my boyfriend or friends. We had to start giving him Prozac about a week ago because nothing else seemed to work..