I do agree that that idea can be a blanket statement sometimes. However, there are many cases (and I've known a couple) where someone who probably should be by him or herself to grow is often caught up in relationship after relationship because they are codependent or afraid to be alone. And these relationships are not usually the healthiest. That's one of the reasons why you should be able to be comfortable alone before you invest in a relationship for the wrong reasons. That's not to say that you can't grow in a relationship. You can definitely learn a lot about yourself, if you're honest with yourself.