Usually when we make assumptions about someone we justify it by saying we're thinking of them and their feelings when we really are only thinking of ourselves. We justify it with excuse after excuse. How often do you say "Well, let's not tell so and so as it would just upset them" when what you really mean is that telling them will make your life more complicated so by NOT telling them you're not sparing their feelings but simplifying your life. Or how about "I didn't ask you because I figured you'd say no" which really translates to you were not the person I wanted to do whatever it was I was doing with. It's simply the wimpy way out. Then there is "I didn't want to hurt you". This is the granddaddy of them all whether between lovers, siblings, friends or whomever. If you did something, made a decision or told a lie so egregious it would hurt someone, most likely you were thinking of you, not them. It's really all about being selfish and that's OK , just don't kid yourself
I've figured out after many years most people are stronger then we think, tougher then we give them credit for and generally speaking handle most things with a great degree of maturity and dignity. To avoid making our own lives more difficult, we simply make assumptions about those we care for, and care for us, to have the ultimate built in excuse. In the long run it's best to take these things head on, simply be 100% truthful and let the chips fall where they may as short term inconvenience, discomfort and hurt are far better then long term hard feelings, ill will and regret.
Is it easy to do? No, not at all. As I originally stated, it's human nature but I think if we try just a little harder we'll all make less of an ass of ourselves