I think you can love someone else without loving yourself. Some people just don't think about loving yourself (even those two words), some are probably disatisfied with how their life became or was, but can still love someone. Do I only think like this? I think that quote's such a trite expression. My opinion.
The danger with that is that you risk depending on the other person to make you happy. Your happiness and sense of worth hangs on the mood and words of your partner. It will send u on a rollercoaster of emotions since noone is perfect and say bad things from time to time. My sister was like that and i watched her ruin her marriage even tho she had a husband that loved her and a child.
"if you're not able to love yourself, you cannot love anyone else"
Respect for the mangaka. Bcoz of their manga makes me want to learn to love my self more ... hope there's another 'masahiro' for me XD