Why does my dog love to pee on beds?

My dog is a Maltese poodle. He is 4 years old. We trained him very well how to go outside when he wanted to pee. We would leave him inside the house and he wouldn't do a thing. Then it started about a year ago. We started leaving him outside because he would pee on every bed he saw, he didn't even have to pee you can tell he just wants to make us mad. We hit him we scream at him. We have a guest house if you leave that room open for one minute and your not there it's already peed on! What do I do?!!! He drives us crazy !!! Is it because of his breed or wants attention or what? Why does he do these things is my main question.