I think a lot of it is also trust. I trust that anyone worthy of being my friend is someone who I can screw up things with from time to time and it won't count against our relationship too much.
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I try to expect it's much more serious then the extent people talk about something, because in my experience what people do say is the tip of the iceberg. People can be so shocked like there were no signs, but they were there all along, people just didn't take them seriously enough.