It is possible to be polite all the time if you never say what you really think. If you want to be polite all of the time you just smile and nod no matter what anyone says. Don't forget your please and thank yous but pretty much avoid saying anything else. You will never offend anyone. Of course, nobody will ever know what you really think or feel either.
Being polite all the time is certainly possible, although it can be challenging in some difficult situations. I believe having the right attitude is key to being polite, because every situation can be viewed differently, entirely depending on your attitude. With a positive attitude, being polite all the time is certainly possible.