In my case I know a lot of the external issues, most notably this scenario...
Multiple relatives dealing with advancing age and/or severe illness where I am needing to step in and assist. I get it, it's the circle of life. But then there's the side factor that my wife and I are childless, and not due to lack of trying, and all the trying (i.e. engaging medical science up to the point of multiple IVF attempts) caused cascading medical after effects that by the time we handled them we were told by adoption agencies that we would be long shots at best to adopt a child due to being older now. So I'm handling a lot for my mom for example, which I see as part of the family social contract, but at the same time I know my wife and I will have none of this kind of support when we get old, and that's depressing. Add onto this that we're in the holidays and going to family events where we're seeing all the nephews/nieces, and now some of them having kids, it just highlights this large hole in my existence that I feel should be filled, but can't be by the normal means (having kids). So I lean on my distractions/hobbies/pets. And now those aren't doing it anymore, or if they are "doing it" it's to a considerably lessened degree.
I know that hormonal balance and biochemistry isn't going to solve that problem. But I also know that when I was rocking the T level of a 22 year old my focus/drive/energy level was as such that it was at least easier to motivate myself to do things, and in general I just felt better. I also know that for the most part anti-depressants are just going to "narrow the emotional range" where you feel fewer lows, but also feel fewer highs. That doesn't feel like a good solution for me.