For me it was many things, the biggest one by far was always feeling like an alien. I have broad knowledge of many different subjects, which is unusual apparently. I also used to have terrible social skills (not being able to read the room, monopolizing conversations and so on), my emotions are basically 0 or 100, with little in between.

Those were the main things that led me to seek answers to the famous question "why am I like this ?"

Afterwards, everything clicked together, and I discovered how deep my ADHD and ASD actually were. It explained pretty much every single aspect of my life, and layer by layer, I discovered hidden things that I could now explain (constantly feeling tired, hating the world when overstimulated, dopamine spikes and crashes, etc).

It obviously takes an enormous amount of time (especially with late diagnosis, like mine) to understand yourself completely and undo everything, but it's so worth it.