Before I thought this statement wasn't true but only because I was in love and what that relationship made me realize is that this statement is so true.
you can't love someone until you love yourself because if you don't love yourself and get into a relationship you are going to rely on their love and convince yourself that you love yourself to because you love them and they love you. But if they were to leave you are going to fall apart because you relied on them for self love and happiness. That's why before you get into any relationship you need to know your worth.