Why are some people so arrogant that...?

Question

... when you politely but firmly tell them your decision about something, they start acting snotty?

It's not like someone owns me - I'm 25 and old enough to decide everything for myself.

Answer 1

They are immature and think that if they whine and act like a baby that you will change your mind about your decision.

Answer 2

You said it yourself.  They want to "Own" you, and you turned them down. Net result: They get snotty.

Answer 3

That's not necessarily arrogance, it might just be bad manners.  When someone insists on forcing their opinion on you, the best thing you can do is remain polite and say "Thank you for taking the time to share how you feel" and then move on :-)