Random Thoughts.....What Are You Thinking?

He was phased out by staying with a Pokemon researcher named Professor Ivy who was a woman, probably in her late 20's to early 30's. When he returned, the mention of her name sent him into depression which sparked rumors because not many people picked up on WHY he was depressed. Rumors included a sexual relationship between the two that Brock couldn't fulfill due to inexperience, Brock got Ivy pregnant, Brock and Ivy started dating and broke up, Ivy ended up having a lover, etc.

In reality the real reason can be found just by looking at both characters.

Brock, in his debut, was shown to be raising his siblings after his mother left and his father disappeared after a failed Championship. He's used to being responsible, keeping house, and is pretty much known to be tidy.

Ivy on the other hand, as shown in one of her episodes, lives in a pig sty, is very messy, and also has three assistants living with her, none of them leaving time outside of research to clean house.

The likely answer is that Brock began living with them and while infatuated with Ivy, she and the assistants used him as a housekeeper and he constantly had to clean up after them. In the episode Brock returns, he is running errands and helping out Ash's mom, thus further establishing himself as a busy-body.