Hobbyhorse is a noun that refers to a topic that someone talks about frequently. 

Dating back to the 15th century, our word of the day came from Old English as a reference to a toy given to children. In time, the word developed a metaphorical meaning and came to refer to a topic that someone is greatly preoccupied with. Here’s an example:

Michael’s hobbyhorse had always been the swimming pool at the park. For as long as I can recall, he always began his day with a long-winded rant about how nice the pool was until the park board stopped cleaning it up.