relationships tip to one end or the other. Even the most perfect relationships whether they are best friends, lovers, whatever - one partner always cares about the relationship and puts more effort then the other. In turn, the other has control on the relationship and where it goes.
But in most instances, the difference is negligible. Meaning it isn't an extreme of one person putting more effort then the other. Just a tiny bit here and there. Tiny things. Like maybe one will be more keen on sending the other funny pics and the other just doesn't. Or one is constantly sharing funny YouTube videos for them to both enjoy.
But a one sided friendship is a typical constant and clear cut disregard for the other person. Like they act great together they get along in person and even in texting and calls. But it's the actions that define it. For instance, hospital visits.Two friends are in hospitals for different things at different times. One visits and the other can't find the time or the energy to even bother to call. Basically, a friendship of convenience. When THEY want something they'll call.
I had a friend like that until almost exactly a month ago and then I railed on her for three hours about what a terrible friend she is. Told her that until she's ready to put effort into her friendships as she does her ego, not to bother me.