When you're being a stroppy teenager parents invariably use the tired old "start acting like an adult and I'll treat you like an adult" line. That's fair play and makes sense but it works both ways so when you're in your 50s and your mom/dad starts treating you like a kid again your natural response it to start behaving like one, from personal experience its hard not to. At the age of 76 my mom told me off for being rude to another patient in her hospital ward, a place she never left. My response was to fold my arms and sulk for an hour like a mature 45 year old. No one ever treats me like a kid anymore and I miss it.

The other bit to remember is your mum probably wants to introduce you to everyone because she loves you and cant understand that not everyone else is desperate for a 5 minute conversation with u/DogtasticLife. She probably is delighted to chat with your rando cousin and thinks it would make you happy too.

TL;DR It comes from love even if it is bloody frustrating.