Educate Me: Markey vs. Kennedy

Massachusetts Rep. Joe Kennedy (D) is challenging incumbent Senator Ed Markey (also D) in a primary. Recent polling suggests Kennedy is ahead.

Can someone explain to me why this primary is happening? From what I can tell, Markey and Kennedy have relatively similar views. Markey is a staunch progressive, but Kennedy is not challenging him from the center. There's no scandal I know of in Markey's file either. It seems that the motive for Kennedy's challenge is nothing more than "I want to be a bigger deal than I am now", which is not exactly the most inspiring rallying cry for a primary campaign. Yet he seems to be winning. Is it just the magic of the Kennedy name in Massachusetts?

I have a pretty high bar for supporting primary challenges against Democratic incumbents,* and things like "we need a fresh face" or "generational change" virtually never qualify. If there's anything to Kennedy's challenge other than an ego trip, I'd love to hear it.

* Recent examples of challenges that passed this bar include Marie Newman over Dan Lipinski and Jessica Cisneros taking on Henry Cuellar.