Companies love feedback on their products. I've received many coupons from different companies over the years just for complimenting them on a product I love, or on occasion, complaining about a product or service when I've been dissatisfied.
I emailed Bumblebee Tuna about a week ago, telling them I love their product (we eat tuna a lot here) and asked them if they had any coupons available.
Guess what I received in the mail today! A very nice letter thanking me for my comments, and 4 coupons for 25¢ off! (These can be doubled for 50¢!). I'm a very happy repeat customer!