Practicing mindfulness meditation has been great for me in regards to this. One recent revelation of mine was that I'd instinctively react to almost everything my brother said with a slight distaste (we've bee living together recently). When I became aware of this and inspected it further, I found that this was because I subtly thought I was better than him. I was never openly patronising or elitist - I'm quite contentious of my outward communication - but there was definitely a mental state there. Of course, I saw how silly and potentially damaging this was and on letting that assumption go our interaction has been all the better.

I think the key is giving yourself space from the thoughts and emotions of the moment. When you assume someone has attacked or inflicted some negativity against you, its very easy to get carried away with your instinctive reaction. Especially so on the internet, where there's a lot more room for mis-communication. Also, being mindful to explore potential perspectives other than your own. Assumptions mostly come from us imparting our own experience, whether it be lacking or greater, on to a subject that doesn't warrant/fit it.