I guess my response to that would be twofold, and I'm definitely not trying to argue with you on it, fyi. One, there are an infinite number of challenging things that a human can pursue, so you've got to prioritize at some point. Juggling and learning new languages are challenging too but they're not really relevant to the theme he's chosen. And two, I'm sure he does have many successful relationships, so why would the single romantic monogamous relationship that you're referring to be more important than any of those other relationships? The later point is particularly important to me as I'm a single guy in my late thirties and I utterly love everything about being single, despite the social pressures to the contrary. Just my take....
The 2nd time I listened I felt I heard a lot like of holes in his thought process. Like if he goes after what's hard why isn't he chasing a successful relationship. That may not be difficult in the physical per day, but having SUCCESSFUL relationships is one of the most difficult things.
All credit to the guy though