I don't think it breaks down at the macro scale so much as humans seem to have an innate need to identify with groups more quickly than as individuals. I've always been a bit of a loner and not much of a joiner, so I tend to identify as an individual first before a member of a group. I am fully aware that this puts me in the minority. If people identified more as individuals first than as group members first, I think we'd see more deference to humanism on the whole.
I have the same kind of problem with your other statement, we define ourselves by what we oppose. Again, I find myself in the reverse here. I am opposed to things but I don't lead with that, in general, and don't define myself in that manner. That is, again just me.
If this truly is how most people think, no wonder I find myself so out of phase with the current reality.
That said, being opposed to something, it does not follow that you are necessarily for anything, being for something really forces you to be explicit about what it is you are for. Again, I believe if people defined themselves by what they are for, we could go a long way to preventing misunderstanding. It's one of the issues I have with modern feminism, in that it spends a lot of time telling people what it is against, but it doesn't ever explicitly give us the "for", and there is an end-game to any movement, an end result which represents the "for". So we are left to wonder about the "for". In the absence of a clear statement of the "for", and the fact that goalposts for inclusivity move around nearly as fast as you can run after them, you start to wonder if anyone really knows what it is all about, or if it it's about anything specific at all.