How many times do we have to say hello?

Walking through my offices today I passed the same co-worker several times. The first time she gave me a very pleasant hello, and I returned the salutation. Then about 5 minutes later our paths crossed again, and as if it was the first time she had seen me today she said hello again. No big deal. But then we went passed each other again, and again she said hello. Yeah! Hi! I acknowledge your presence in the building. You're here and I am here. Let's move on!