Saying Thank you

Why is it that to get my two and a half year old to say a simple "Thank you" is like pulling teeth. She is seriously so suborn. I just have to wonder who she got that from...(and idea's)? We sat in time out today just for not saying it. But then there's little Heath. He is thirteen months old and has been saying "Thank you" over a month. It seems to come natural to him. I had a snack and in his little gibberish, you hear it. Oh, I know that people thought I was crazy when I told them. But Hal heard it a couple of days ago. I could still be crazy, but not as much if he heard it too.
Haley can be really sweet. She will take Heath his cup anytime I ask and always offers him half of her own snack. I asked her which cup she wanted and she said "Mickey Mouse, please." So at least she has the please down. We went for a family walk with the dogs the other day. Hal had Heath and well, the boys got quite a bit ahead of us. Haley called out "Daddy, Heathbar...no." I really think she got upset that Hal took her brother out of her sight. Or as Haley refers to him "Heathbar."