I mean I've heard people say things like
I dont know why But it doesn't sound idiomatic or grammatical to me.
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It is very idiomatic in informal speech to leave out a pronoun at the start of a sentence if it would be clear from context. The practice is called subject pronoun drop or diary drop:
Sentences in English are generally considered to require overt subjects. In a standard description of English, (1 a) below would be considered grammatical, while (1 b) would be considered ungrammatical:
(a) I walked the dog yesterday.
(b) Walked the dog yesterday.
But in fact this is not true. (1 b), and sentences like it lacking subject pronouns, are in fact common in colloquial spoken English. This phenomenon has a written counterpart in “diary drop” (Haegeman, 1990, 1997; Haegeman and Ihsane, 1999, 2001); subject pronouns can frequently be dropped in certain registers of written English, such as diaries but also including text messages, emails, and other forms of informal communication...