"Is it easier to write poems about sadness?"  
Yes and no.
It's easy enough to let out insanity in a wolf pack of words racing across the page. It's easy to be sad, to write yourself off a cliff or into a void.
It's hard to have the courage to own that emotion and move past it.
It's hard to catch a moment of unspeakable beauty without it crumbling, like capturing a butterfly with your bare hands. It's hard to find them and embrace them you have to seek them out, but I think in the long run they are better for you.
At my lowest low I could never have imagined the joy my life could one day find, but I'm glad I waited.