Come out and tell her how you feel. This can be tricky. If you want to tell a girl/woman how you feel, start off smooth and tell her what you want in a relationship, tell her why you want her, and tell her how you would treat her.
Usually because he is sensitive, or because he isn't sure if the girl feels the same way about him. Most guys feel it's almost the girl's job to tell them how they feel about them first. Most guys aren't sure how to approach a girl and tell them how they feel.
Well this could be a problem because the girl might think you want to PHYSICALLY express how you feel about her and she may feel uncomfortable with going into that so fast. Just take her to some place where you guys are alone and tell her how you feel.