Besides that warm fuzzy feeling you get from being nice to people. Good customer service can make you or your company lots of moneyz. By putting in a little effort you can bring in more customers without so much advertising. Advertising can be expensive so you want to keep the customers it brings in.
How enjoyable is your work. If you just put a little bit of effort into what you do, everything will be more enjoyable. Being friendly will create a positive atmosphere. Even something simple like being organized to better suit your customers. Everything you do extra will keep you busy and make you and your customers happy.
Motivation is key. If you can motivate yourself to keep giving oh so great customer service your company will profit. You can also try to motivate other co-workers around you.