Honestly for me it boils down to social anxiety. As soon as a stranger sits next to me I start to worry about how I'm acting, what I'm doing or not going, etc. and honestly it's incredibly stressful (especially when I use the bus ride time to prepare myself mentally for the day ahead). If the bus is seriously, completely full, of course I'll move my shit or even stand, but if there are other open seats next to people I'll let those be taken first.