Well, I suppose from their perspective you, your DH and your existing kids are not 'new'

My mum always made me laugh when DS was tiny. She lived interstate and whenever she'd visit she'd be so excited to see him that she'd rush in and start hanging out with him without a thought for anyone else. After about five minutes I'd say 'Hi Mum', and she'd look up, realise that the rest of the universe still existed and get all sheepish. But it was hard getting mad at her about it because her excitement was so infectious.
My mum still does this
While it frustrates me I do get it and why people do it and most of the time it doesn't worry me but other times it frustrates me.