because while loving someone, the biggest mistake we make is that we depend on them for our happiness. we forget our individual existence and sometimes we even loose our identity. By loving someone we make the mistake of giving them the right to hurt you, torture you and make you fell inferior. This happen only in one sided love. If the couple has true love this won't take place and both will mutually respect and support each other. And as its rightly said even small innocent words from our loved ones hurts us the most sometimes compared to the harsh word and nasty comments from outsiders.