putin's ego basically. yeltsin was much more friendly but welp, we haven't really had any other choices besides these two the last 30 years. medvedev is not even worth mentioning. it seems we had a brief window in like 2000-2001 where putin was genuinely open to the idea of joining nato but iirc the problem was he didn't want to go through the normal application process and wanted to just be speed-lined in, which doesn't really work because nato has shared arsenals etc and you can't go from having 0 connections to any of the allies to being part of that overnight.
United States