Because if I like this girl, and we are together alone, I will probably be awkward and uncomfortable, because I am very shy around women that I like.
However, when my friend is around, I feel much more comfortable, because I have known him for years. If i start to become uncomfortable, he jumps in and keeps me from making myself seem stupidly shy.
Showing their "sensitive side". How true it is and how much of an act, no idea.