This is why, after two months with our first foster greyhound (our Boo), we had to keep her. We were in love with her, after seeing her grow and develop a personality for the first time in her life - and she is still so devoted to us. After that I said no more fosters - only because I'd be afraid that I'd keep every one.
I have so much admiration for those who foster on a regular basis; I just don't have the guts to do it! We gave up one dog last summer, a stray we had found, after keeping him for two months, cleaning him and correcting his ear infections, neutering him, housebreaking him, training him, etc. Giving him up was one of the hardest things I've ever done, and I only did it because the two dogs we had then (Paula and Buddy) didn't like him one bit, and I had to respect their feelings. "Scruffie" went to a good home and a good family, but I still miss him sometimes.