Origin of the "-y" or "-ie" diminutive suffix to denote intimacy/tenderness? (E.g. Bob→Bobby, dad→daddy, Doug→Dougie)

The "-y" is a suffix for forming diminutive nouns, and Wiktionary has an entry dedicated to it, though it doesn't say anything about etymology other than "from Middle English and Scots". Thankfully, The Free Library provides lots of further insight, but the bottom line is that nobody really knows for sure, and "the etymology of the diminutive suffix -y, -ie [will] most certainly remain controversial". The same suffix exists in modern German, where it is spelled as "-i" (Mami, Papi, Mausi, Steffi, Susi, Schumi), but again, the connection to English is unclear.

According to C.A. Ferguson, this is one feature of "Baby Talk Register" or "parentese", a kind of language that some parents slip into when talking to their child at a young age. Parents, for example, might refer to themselves in the third person like "Who's mummy's little boy". One other feature is adding the /iː/ sound on the end of words, like "dog" -> "doggy"/"doggie". I'm not sure if this is something parents pick up from children or the other way around, but either way it's possible that this pattern of adding /iː/ came from this feature of parent-child interactions.

In ancient Hebrew the "i" (yod) is added to denote first person possessive as in "abi" and "immi" (my father and my mother). It appears that possession implies intimacy in a relationship and thus a way to make a name more precious and personal.

Refer to a Hebrew language primer, for example: http://www.hebrew4christians.com/Grammar/Unit_Seven/Prepositions_with_Suffixes/prepositions_with_suffixes.html