Why Does My Friend Need to Know Every Detail About My New Relationship?
Humans are biologically wired to gather information because information used to mean survival. Your friend grills you about your relationship because of: 1) social comparison theory, they're measuring their own love life against yours whether they admit it or not 2) gossip functions as social currency, details about you are currency they can spend with other people 3) vicarious living, if their life feels stuck they borrow yours for excitement. Answer what you want, keep the rest yours.
What's actually happening
New relationship starts, questions start immediately
Small details get repeated to other people
Comparison creeps in, subtle digs follow
Your privacy becomes their entertainment
Myth vs Reality
Myth
They ask because they care about you
Reality
They ask because your life fills a gap in theirs
Maybe they just miss having their own story worth telling.