Why do third parties notice "signals" better then the person involved?

I'm just curious as to why it always tends to happen, two people are intrested in each other and third parties can see oit (friends, co-workers etc.) but not the people involved I've been the recipiant of this i have also been the person that "see's" it I'm just curious as to people's opinons as to why this happens

Put the internet to work for you. via Personal Recipe 2629979