Plath's life was fairly unhappy, despite her achievements. Often, she wrings out this unhappiness and distills it into powerful, sometimes shocking poetry (I still remember the gasps of my students when we read "Daddy" out loud). And while I sense her desire to be empowered and destructive on her own terms (something she achieved in her poetry, since Ted Hughes is largely hated by feminists everywhere, and has been widely blamed for her suicide), I also sense a deep and abiding dissatisfaction with life. And this is blame that is not Ted's to own, but likely due to some deep unhappiness that Plath's psychological treatments never healed and maybe even exacerbated, given the technology of the period. Still, Plath's story and angry poetic confessions are the battle hymn of the wronged woman, and Hughes certainly did her wrong by fathering a child with another woman while still married to Plath (with whom he fathered two children, Frieda and Nicholas)
Once, when I was going through the poems I was teaching my Comp II students, I mentioned Plath and then Anne Sexton, whose poem "Cinderella" I often use in class. There may have been another poet mentioned as well, but I don't remember specifically who it might have been. I was putting my lecture together and making some casual comments as I worked, when Michael stopped, put his hand on my arm, and asked me, very solemnly, "Maybe you can include some uplifting poems?" He was right. It seemed like every poem I had in my lecture for two solid weeks involved grim disillusionment or outright references to annihilation. Sometimes, though, these have the most powerful language and strongest realizations about humans and about life. Nevertheless, I included some Ogden Nash and some happier Robert Frost (not the "lovely dark and deep, mind you....and definitely not the poem about the clattering saw) to keep things light.
I suppose I use the darker poems because I want to impress on students that poems are usually not about butterflies and flowers and cherubs floating on clouds. They can be serious and emotive (like Wendy Rose's "Three Thousand Dollar Death Song", which floored me when I read it the first time).