XH told me years ago that he used to have times when he'd be really depressed and he never understood why during his teens. He said it would last a day or two, or sometimes a week or two, and then it would be gone. He said he got used to it and learned it would pass.
This was my ex. He used to call it "falling into the existential void". Happened a couple of times in our twenties, he'd be "blue" for a month, then go back to being his usual very energetic (I now see, hypomanic) self.
In his 40's, the lows got lower and the highs started to get more impulsive and irrational. But I was so used to it all by that time, I really didn't see the bipolar part of it until the end. Now in retrospect it seems obvious that he had a mild case. But since he spent most of his life in the very effective hypomanic phase, everyone just saw him as this super-effective person.
