To any personal trainers both new and old - getting to know your clients on a deeper level than just fitness is such an important point.
If you are to be a successful personal trainer you should see yourself as a coach. Unfortunately many mistake the job as just providing proper exercise technique. This skill - although it should of course be there - ranks pretty low in the pecking order of skills required to be a good coach and if this is all you are really offering will meanclients come and go on a conveyor belt. Unfortuantely these types of PT's will spend the majority of their time looking for new clients and never get down to the nitty gritty stuff.
Knowing a client and developing a relationship is not only going to help towards keeping clients longer - it will also mean you can actually coach them better as you become more empathetic towards whatever trials and tribulations that occur during your time together. The other seemingly obvious point is if you can retain your clients for long periods of time they will surely see better results.