The young Albus Dumbledore was a brilliant student, respected and loved by his peers, which isn't that from how it was for him when he was older. His love for Muggles was very obvious to everyone, so it was quite a shock to read his letter to Grindelwald about how they should go about taking over the Muggles. We see throughout the seventh book, Harry questions his faith in Dumbledore and whether he really knew him. But Harry really shouldn't be judging Dumbledore for something he did when he was younger. When we're young, we make stupid and foolish choices that we later come to regret. We've all done something when we were younger that we regret and ask ourselves,"Why did I do that?"
Dumbledore is no exception. Harry seems most frustrated with the fact that he knew really nothing about Dumbledore and that he had put all of his faith into him. We all mess up in life. We all make decisions that we later regret, so why should Harry act like Dumbledore is any different. Should we really judge Dumbledore for his decisions as a young man or should we realize that everyone makes mistakes and just except Dumbledore as the lovable old man we knew him as?