In all her years, she has collected virtually every lesson on friendship… except one. She doesn't know what it is, and it bugs her to no end every single day. She tries to find it using her students; she tries to fill the void with the new friends she finds every 20 years or so, but she can't make it last. Ironically, the last lesson is to stop trying to make it last and enjoy what you have while it still exists

And over here is where she keeps her husbands:

It sucks being immortal. Poor Celestia. :(