Non-polar drugs will generally cross the blood brain barrier more readily. If a drug is too polar it will have difficulty or perhaps never even make it. Polarity is also issue when trying to make your way to the blood stream. The benzene ring of drugs is fairly nonpolar so it's a good basis to build up from.
Aside this if a drug is working to inhibit/activate a site it's a good idea to work up from what naturally interacts with it. Take for example the neurotransmitters dopamine, epinephrine, norepinephrine, melatonin, and serotonin. All of them contain a benzene ring as part of their parent structure. If you want to alter their transport, receptor, metabolization, etc. they are examples of what works/fits the site. This is where pi-pi interaction that Arkcon mentioned comes into play, though it is not the only possible interaction. Some drugs can exist without this but for many it's a strong foundation.