Where do you live? In places like US, where healthcare is not socialized, it's common for the dentist to recommend a filling even when you could do without. In that case getting a second opinion might be worth it. Dentists in general though are competent enough to take bite/dental balance into account when filling cavities or putting in crowns. 

I have had several cavities between my molars for years, and apart from aching when brushing directly, they have not bothered me much. Apparently their progression has stopped completely, so I'm not going to do anything to them before they start aching around the clock.

Do everything you can to save those teeth using natural methods. Putting fillings into teeth is an irreversible (for now) and destructive procedure and really should be a last resort. The materials they use for those fillings can be very bad for you. Also you have to keep in mind how that doctor makes money as Progress said. I've read stories of dentists who have quotas to fill of how many crowns and fillings they have to place in a day so that they can pay for their homes and cars etc.

Thankfully, teeth can be saved through proper nutrition/diet and proper hygiene. Most people get the second part right but not the first. If you want to learn how you can start with the links I placed in the nutrition thread on this forum. Read the book and the website I linked to. They will teach you everything you need to know about nutrients and proper diet. Or just go on google and search for "saving teeth through nutrition" and start reading the different links and see if any of them make sense to you and sound worth trying.

Learning this type of stuff will be a complete paradigm shift that may change your idea of what the human body is capable of so I cant just give you a short and simple answer. You have to go and discover it for yourself.

Last thing, if you are interested in dental health at all I can recommend a book called 'Mouth Matters' that talks about everything I just did but with evidence and science etc. The person who wrote this book has some interviews on youtube which might be a good place to start if reading isn't that exciting to you.