Can be lack of boundaries, can be one side’s need to control the other. Lots of reasons why friendships can be toxic. It depends very much on the people involved.
I think one of the most important things in good friendship is to be relaxed about it. Don’t expect other people to think and do exactly what you do, don’t take offence as soon as someone does something you would have done differently, accept that people won’t always have you as their number one priority. Things like that make good, lasting friendships, in my experience.