The idea is close to our I am grateful shirt, however, there are 2 reasons we also wanted to add this shirt to our collection. Reason number one is that for some people, when they say the sentences I am grateful and I am thankful to themselves, it feels more natural more correct, more convincing to them to say they are thankful (or grateful depends on the person), whichever one feels more natural, real, correct for you is the one you should choose, cause it will have a stronger effect on you and your mind.
The second reason is that with people who are thankful, people associate others being able to actually tell others thank you. Telling others thank you even for small things is a very strong powerful thing. It will make others feel a lot better, it will brighten up their day, to at least every once in a while hear someone tell them thank you. Making them feel that they are doing something correctly, they are doing something good, even if it is just a small thing.