Taking your partner for granted is often one of the sins that people in relationships commit. You assume that your partner naturally accepts all of you, good and bad, such that you end up not bothering about each other's needs. You do not bother to change your bad habits, the very flaws that drive your partner to tears, because you think, when we first started going out together, I was already like that so you have no case to complain. And the worst thing that happens is that when said partner, who was driven to tears by the things that you do, end up leaving, you still do not learn from your mistakes and change.
You did not listen when she said that she felt like she was performing a monologue when she talked to you. You thought that she was complaining about nothing, making a mountain out of a molehill. You told her off, saying that this was who you were, without putting yourself in her position. You thought, she doesn't understand me.
And then now, through someone else, you come to realise that you are a bad conversationalist. And you thank this person for helping you realise this truth.
And hence, unknowingly, you break someone else all over again.