If you patronize a business, you shop there regularly. But if someone patronizes you, it's not so pleasant — they talk to you as if you were inferior or not very intelligent.
Patronize comes from Latin patronus "protector, master," related to pater "father." So if you patronize a person, you talk down to them like a father might do to his child or a master to his apprentice. If you want to take an advanced class and your advisor warns you of all the hard work, you can tell him to stop patronizing you — you know a hard class involves hard work. This sounds much better than saying, "I'm not stupid!"