I would say the person that is too shy to share with another that they love them could be the fear of vulnerability. But why would anyone want to deal with a person that can not show or reciprocate the ultimate beauty of love. Sharing Love with one another is the most wonderful thing in the world.
Because when you admit you love someone, you open yourself up to rejection. And when you admit love you're basically telling them they have gotten to you, and that you are vulnerable to that person...some people have a hard time showing this.