The best relationships are when both sides invest equally in it. Both sides equally care.
This is true, I think, for any kind of business partnership, but I’ve noticed it particularly in personal relationships. When one person is way busier than the other, it’s a weaker bond. When one person is way less interested in the long term resilience of the relationship, it often wavers.
If one side thinks a relationship receives equal mutual investment, and then is proven wrong, it can be quite hurtful. This happened to me once. I misjudged how invested the other person was. I thought we were closer than we actually were (in the other person’s mind, at least). It hurt.
There’s a reason why the term is "best friends" and not "best friend." If one person considers you his/her best friend, and you don’t think that way about him/her, then you’re not "best friends." It cuts both ways.
(On a somewhat related note, 25% of Americans have no one to confide in.)