To cringe a little bit when GPs say 'thank you' for grandchildren?

[IcouldstillbeJoseph]

I understand the sentiment of course, i just feel this is a slightly weird thing to say.
Why 'thank you' as if it was a present, just for them.
Maybe it's just because my PIL are vile said this...

(nibbles delicately on a chocolate drop sniffs a sweet blossom and decides perhaps its time to leave this place)

Now that's more like it.