Agree, OP. We had a nanny who at two weeks I was ready to practically adopt her. She had only been with us for four weeks, and my husband and I were literally discussing how many weeks bonus we could give her for the holidays considering how much we adored her versus how short she had been working - when we received a text message from her, quitting on the spot - she was moving out of state to be with a boyfriend. Didn't give us any notice whatsoever.
Not only was it really difficult of course to find back-up care, but I think because we got so emotionally invested so quickly, it really was a blow to us emotionally too. We really really liked her and thought it was mutual, but obviously, we didn't know her at all yet.
Be happy, make sure she knows she's appreciated, treat her fairly, but also, slow down
