Does anyone know what causes burnout?

It’s like you’re super engaged and feel like you could do x forever until you suddenly crash. Then you’re bored for either a second or like a month (no rhyme or reason but the chaos in front of you) until the next thing comes along. But Godamn that boredom is real and often leads to sadness. I need a why to the whole thing

Christ I don’t know if this is relatable, but it explains a lot of my problems be it people I’m obsessed with for a sec and genuinely think I love or a different type of work assignment or a hobby of some kind