"Melanie Klein (...) argued that when children are small, very small, they don’t really realize that a parent is one character. They actually do what she called split a parent into a good parent and a bad parent. And so this is when a baby is really at an infant stage. So what you do is you split into the good mother or — and the bad mother. And it takes a long, long time. Melanie Klein thought it might be until you are 4 until you actually realize that the good and the bad mother are one person and you become ambivalent. In other words, you become able to hate someone and really go off them and at the same time also love them and you are able not to run away from that situation. You are able to say, “I love someone and hate them and that’s okay.” And Melanie Klein thought this was an immense psychological achievement when we can no longer merely divide people into absolutely brilliant, perfect, marvelous and hateful, let me down, disappointed me. Everyone who we love is going to disappoint us. We start off with idealization, and we end up often with denigration. The person goes from being absolutely marvelous to being absolutely terrible. Maturity is the ability to see that there are no heros or sinners really among human beings. All of us are this wonderfully perplexing mixture of the good and the bad. And adulthood, true psychological maturity — you may need to be 65 before it hits you; I’m not there yet — is the capacity to realize that anyone that you love is going to be this mixture of the good and the bad."
(Why You Will Marry the Wrong Person | Alain de Botton)