Some of what he describes is relatable, but I think his mom had a point about these being common human feelings. 4s don't experience clinical depression necessarily; it's more of a demeanor and mentality than true depression.
The guy's demeanor here is unrelatable to me. His manner of expressing doesn't feel familiar, but then this is obviously a rehearsed or "acted" monologue by him, not a spontaneous, vulnerable soliloquy.
I do feel times of pure, unadultered joy. I don't immediately have a negative voice in my head that counters these experiences and feelings. However, a fantasy or expectation may grow out of it, and then when it goes unfullfilled, the original positive value is tarnished. It now is just a dashed hope, a false alarm, a promise that led to disappointment, instead of being able to exist as something good in itself. But in the moment, I can truly feel a good emotion, and it can be very strong (whether I display it or not); I would say that I swing from elation to despair rather quickly.
Some of this may be an auxiliary Ne issue - things are good when brimming with potential, when possibility seems on the verge of expression. The actual results always pale in comparison.
Once I fulfill something, even satisfactorily, I may then hone in on something else that seems lacking. I feel an inability to find balance. If one area of life is going well, then another seems to be a mess or just empty.
Or I may have thought something was good and truly felt it at the time, then later I may think I was delusional (ie someone seems to like me, but later I seem it was politeness at best). This can make me distrust good feelings, so once their initial high wears off, they seem less "real". The sadness seems to reveal something deeper about reality, and so it becomes the basis for interpretation, the overarching pattern. Something is more reliably real when sad. I think a stereotypical average 4 attitude is that happiness is kind of shallow and fake, and this may be why. I think this is probably poor Si in an NFP also. It is the idea that sureness and constancy is sad, suffocating, etc, and that fleeting, fickle exuberance that suggests something (but may never deliver it) is where positivity lies.
In order to keep that forward momentum to stave off the sinking feeling about reality, you have to have frequent change to provide context for new potential. But to realize potential, you often have to put in steady effort. That's where I may fail, because the steadiness I associate with the sureness of disappointment, and I lose motivation, and then the fantasy never manifests. Then all the intitial positive feeling is tainted....it's all part of one larger experience of disappointment.
To refer to the video....the guy thinks someone likes him, but then later decides it must be that they pity him. My interpretation of things can swing like a pendulum because the initial experience does not go anywhere, or it may seem to go downhill. That will make me think my initial estimation was a delusion, and that the larger pattern shows this.It probably takes an excessive amount of repeat positive results for something not to get soured for me. I don't tend to discount negative experience so readily, however.