"To love one another" is general, referring to any people. 'To love each other" refers to two people. However, "to love each other" is used in the sense of "to love one another," so you need to be aware of the context. "To love one ather" would never be used of just two people. I love you; you love me; we love each other. I love everybody in our family and they love me; We all love each other; we all love one another.

Hmm...I'm not 100% sure, but to me it sounds like the first is talking about a group of people (for example why is it so hard for mankind to love one another) while the second sounds like two people (a couple having trouble with their relationship?) Hope that helps! Do you have any other context?