Why is it hard for Aquarius people to let someone in?

What about us aquas... makes it SO difficult for us to let anyone in? Especislly when we have feelings towards the person, usually people let another in. But with aqua's we can't let that person in. Especially if we have felt severe emotional pain in our past, its like we built a rock and that rock devolps into a boulder making us unemotionally available. Why is it hard?