Not to get all mushy but:

You aren't losing "daddy's little girl", you are teaching her that her decisions are important and that you trust her judgement. Remain her friend and keep up the communication level. Growing up to think for one's self is one of the best things you will impart to your daughter.

Wouldn't it be cool if in the future, she desides to test out a would be suiter by taking him fishing with her dad?