With writing, you either say something or you don't. You have to express emotion to get it across. You can order your thoughts, think about what you want to say. There is no dreaded 'middle-man' (only expression I could think of) of the actual physical human themself - there is no eye contact/gesture/expression/tone of voice to misinterpret. The words are always going to be there - it's harder to get confused.
Hope that helps.
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I am just a broken machine and I do things that I don't really mean.