When I installed my sims2 it came with nightlife when i installed pets it took away nightlife, what happened?

Also when I installed another expansion pack (apartment life) it took away pets and gave me back nightlife. I'm confused. Now I can't play any of the families I made when i had pets because they all have pets. Please tell me how to get back pets and still have my other expansion packs!When I installed my sims2 it came with nightlife when i installed pets it took away nightlife, what happened?well usually in the some of the expansions, when choosing your house in city mode, there is an option at the top to use other expansion's features.