Because a certain holiday is coming up for us Americans. You lucky Canadians already got it over with and everyone else doesn’t have to deal with Forced Family Interaction Day. How I envy you that.
Thank (the verb, not the noun with the s at the end) comes from the Old English þancian, and since the þ is pronounced th, that means the word is thancian. I’m not sure if that c was pronounced hard or soft, but since it comes from the Proto Germanic thankojan, it seems likely.
Now, the word can be traced all the way back to the Proto Indo European word tong, which means think or feel. And yes, that’s the origin word for think as well. In fact, think has a very similar origin. It comes from the Old English þencan and Proto Germanic thankjan, so think and thank have been only one letter off from each other for what? Five thousand years? As for why a word that means think evolved to mean gratitude, well, that’s a little less clear. It’s just known that by the eleventh century, a word related to þancian, þanc, which first meant thought and eventually started to mean good thoughts. From there, it went to gratitude, and that’s why we have thanks.
Looks like it’s a quick one today. I guess you owe me your thanks : ).
Tony Jebson’s page on the Origins of Old English