Yeah, from my completely unprofessional, layman observations, there's two internal phenomenon that a person will describe as depression.
The first one, like other posters have said, is a chemical imbalance that severely limits that person's ability to function and requires intensive medical treatment to get them to a stage where they're at least able to shower, go to work, etc.
The second is actually sort of like a disordered thinking based on that individual's life experiences and personality, and can't really be treated via medication even though doctors will prescribe that to them anyway. Really what they need is long-term talk therapy to shake them out of their thought cycles and into making difficult changes to themselves/their lives.
I'd guess Ethan falls into the second category, especially since the depression only seemed to become a huge issue with him after he found success and started a family. Even as negative thinking goes, that's pretty weird. Usually people are stuck in negative thought patterns/behaviours because they're poor and alone. Ethan has all his needs met and still has the sads, and I don't think it's at all chemical because even while on the medication he clearly hates himself.
I can't look into his head so I'm purely speculating, but to me he seems like the type of person who thinks they deserve more than they have, even though he already has more than most. He seems like he thinks he deserves Pewdiepie or Rogan levels of fame and his "depression" is really just him realising that he's not actually that smart or funny. Unable to accept that and enjoy what his level of entertainment has got him, he gets "depressed".
Leftovers is a good example. He seemed to think it would be a Frenemies-level hit, not realising that no one wants to hear him talk about politics because he's just not very informed, insightful or smart, and only edgy teens want to hear Hasan.