"I _constantly_ try to evaluate where I stand with people. "
I am, or rather used to be, quite similar. Now I am much more relaxed, as I think I understand better what a gift is. It is free and comes ideally with no strings attached. Otherwise it is an transaction. And it is a bad idea, to mix them up. My default is to consider a gift as a gift and nothing more.
I just give and take freely.
If you think too much about it, it can destroy a lot.
Also, there is more than money. Maybe the one guy bought more rounds, but then he unloaded his problems on the others. Or not. So what? Life is not a balance sheet, that needs to sum up. But if you try to make it one, you might find the joy is getting lost somewhere in those numbers.