persisted throughout her life, and her feelings of isolation led to repeated suicide attempts (one of which she refers to in the poem) and ultimately a successful suicide. Yet, even as she excoriates the "Daddy" in the poem, she longs for him: "At twenty I tried to die / And get back, back, back to you. / I thought even the bones would do."

I think that the theme of loneliness is enhanced throughout the poem.  Plath is able to use the fact that the speaker is seeking to be alone, or to be estranged from her own past.  In the poem, the speaker, presumably Plath, is seeking to rid herself of her own psychological baggage caused by her father.  The theme of loneliness is one that is actually sought, as opposed to being wedded to the psychological trauma of her father, seen in a failed marriage to a husband that resembled her father.  Loneliness is seen as something that is preferable to the condition of being constantly accompanied by the memory of her father, the damage it has done to her, and to being with people who keep her in a condition that her father did.  It is for this reason that the ending is so much a foray into loneliness.  When the speaker says, “Daddy, daddy, you bastard, I’m through," it brings to light that the speaker would rather be alone psychologically and be forlorn in the world than be with these destructive constructs that her father imposed on her or constructs that were perceived as imposed by her father.  It is here where I think that loneliness is advocated and developed in the poem.