The subject in the Japanese sentence is often left unsaid, and assumes that the listener or reader knows what you are talking about. It is preferred not to speak words that are apparent from the context.
From the jisho (dictionary): “In Japanese the verbal part of the sentence is the most important, and normally comes at the end. In a long sentence the listener has to wait till the end of the sentence in order to grasp the meaning. In English the grammatical subject is the most important part, and is expressed at the beginning of the sentence, and auxiliary information about the subject is imparted gradually. This makes for great clarity of meaning, whereas Japanese sentences can often produce ambiguities. But this is a product of the Japanese culture, where reticence is considered virtue and outspokenness vice.”