saman, i just went through a conversation with a friend who was thinking along the same lines you are. I kept saying, "if anything, this should have been explained in the books", because now we actually know WHY Dumbledore's history with Grindewald is so complicated and why he sorta went along with him when they were younger.
Dumbledore never made too many mistakes, but I for one like him more knowing more about when he was younger and what shaped him into the "sentimental old fool", to use his own words, that we get from the beginning of the first book.
if anything, the fact that his biggest mistake was because he was in love with what should have been his enemy makes that big mistake so understandable that it almost seems like an insurmountable cruelty of fate that nobody could have beaten. i mean, can you imagine if you had to either kill or imprison for life the person you were in love with? the fact that dumbledore had to do just that makes his beating Grindewald much more of an accomplishment