Did you read that freelancing book that talks about this? It basically says what you learned the hard way - get it in writing before you even buy the stamps. Saves so much headache.
Spot on. This is why @tyler_henderson is right about it applying to more than work. I see the same pattern with friends helping each other move or borrow tools. People just assume things will work out fine without talking it through first. Then someone feels used or the job isn't done right. A quick text to agree on the details fixes almost all of it. It's not about being mean, it's about being clear so no one gets upset later.
Totally, getting burned by a handshake deal taught me the same lesson. Like @sarahf99 said, that book advice is spot-on about writing it down first. Now my rule is no contract, no work starts, period.