Border Collie who sniffs?

I have a Border Collie who will be 2 in July and we've had him since he was 3 months old. He's the sweetest dog. He is very smart and attentive too. But he is always sniffing, he will sniff the whole house. I understand sniffing people, stop signs, bushes and other stuff out of his territory but why does he sniff stuff that he smells all the time? He'll even lay by the outside of my door in the morning and wake me up by sniffing under it. Or when your using the bathroom he'll lay their with his nose underneath the door and you'll hear sniff sniff, sniff sniff, sniff sniff... it's so weird.