It's easy for us to play armchair quarterback and criticize him for not opening up to her, but we have to understand his mindset. This guy has probably gotten picked on for his height most of his life, so he already has low self esteem from that. He hangs out with her all the time, so he may not have much going on in the friend situation otherwise. Yeah, he likes her, but he can't fathom that she would like a shrimpy little guy like him in return. If he confesses, that could ruin what friendship they have. In fact in his mind it will ruin it because in his mind she can't see him that way. So he tries to hide what he really feels so he can hold on to what he has.