New/Old Friend/Acquaintance

So when I went to Mexico for the wedding I reconnected with an old/new friend/acquaintance. We had conversations about the past and she was very insightful about a few things from the past and present. I just want to know why I, a person who is a pretty good judge of character, didn't see that my best friend from college was fcuking psyc.hotic. Why do we let the glaring signs go when it is someone we love? It doesn't matter the love, we just let it ride. She was really close to my best friend but she let her go b/c she saw the psyc.hosis. I hold on to a lot of people who show big a** red flags. I wonder why I do that. Maybe that is an issue I can work out with my next therapist.