Why are facebook relationships such a big deal?

I jokingly texted my girlfriend after she was playing around with me making fun of me through a text and she called me a creep or something and said "i should probably just dump you" of course she was joking. then i said "fine, then i'll just delete you from facebook" and she FLIPPED out and said "NOOO PLEASE DON'T" and then sometimes i jokingly say "i'm going to delete my past wall posts" and she flips out again, why is this? im 18 and she's 17 and we've been together for 3 months.