Why does awesome mean super good, and aweful means super bad? To illustrate with a true story: A chassan with broken English called his Kallah to tell her how aweful she is. You can imagine how she felt.
Why does brainwash mean to empty the brain of good ideas, and brainstorm means to fill the brain with good ideas? And this time for a slightly exaggerated story: I got up in front of 150 people and thanked everyone for helping me brainwash for ideas. (The exaggerated part is that it happened at the rehearsal. No one was there yet.)