I don't know if anyone's noticed this from Daredevil, but Matt has a character flaw that's worth mentioning. It's that he has this tendency to romanticize his friends and lovers. Always wanting see only the good in them, and when they prove to be less than pure he tries to reform them. This notion is in theory noble, but it brings him a world of pain. It distorts his view of the people around him, especially Elektra.
But because Matt has these rose-tinted glasses on (pun intended), he constantly wants to protect everyone, especially people Karen. I think one of Matt’s many problems that strained his first attempt to start a romance with Karen is that he created this image or idea of her in his head. Of “sweet innocent” Karen who must be protected at all times. Of course, he doesn't know that Karen killed James Wesley, so he doesn't recognize that she is perfectly capable of rescuing herself. Matt never stopped to think that what he was doing is unrealistic, but he wanted to keep this pure good concept of her in his world of violence and darkness, maybe because it's what he felt he needed. In his effort to protect Karen, Matt ended up causing more pain than anything else by lying and pushing her away. When Matt disclosed his secret to her, I was like, "Thank god he realized that he was being a dick and that was not the way to do things."
That's the flaw in Matt's relationship with Karen.
But what about Elektra? Well, with Elektra, Matt literally says at one point “We need to stop corrupting each other.” Matt and Elektra do have a chemistry, but it's a destructive one. She wants to pull him over to her side, for him to commit murder. I think the reason their bond is so strong is that she was (I think) the first person who knew about his extraordinary abilities, aside from Stick. He confides in Elektra.
But some people like to forget that Matt does “have the devil in him”. He has that cold "do what it takes" part to him, something that Elektra taps into. He has that, for lack of a better word, “bad boy” part to him. Elektra wants him all to herself, and I get that she says that he is the only person who has ever believed that she can be good, and I get the attraction of that. But she also wants him to cut off all of his personal relationships, which he does. He gives it all up for her and for being a vigilante, and while that might seem romantic it isn’t. It’s controlling. That's the flaw with his fling with Elektra.
Due to Elektra, Matt cuts Karen and Foggy from his life. He throws away his oldest friend, his own practice, and the girl he loves. All because of Elektra. This is not to say that Matt doesn’t have a choice in this. Foggy was right when he pointed out Matt was the one that decided to do all of this, but he didn't know that Elektra was emotionally manipulating Matt.
The best way to summarize this is with something Charlie Cox said:
“I think what’s really interesting about the Matt/Karen and the Matt/Elektra dynamic is that Matt is, in a weird kind of paradoxical way, able to be completely himself—but different versions of himself—with both of those characters. The Matt that comes out when he’s around Karen is the Matt that he really deeply believes is himself. [He believes] that that’s who he is. That’s how he feels about the world. That’s the kind of person he wants to be… However, there’s this big problem in that she doesn’t know everything there is to know about Matt. She doesn’t know this other side of him and he’s not able to completely be himself with her because of that, because he hasn’t told her that he’s Daredevil.
With Elektra, the opposite is true. She knows he’s Daredevil and she encourages him to really go deeper with that, go further with his darkness, and the violence, and the excitement and all that. He feels, in a paradoxical way, very authentic with her, but at the same time, she’s asking him to tap into something that he doesn’t feel is quite true—a darkness, and a violence, and a disrespect for law and order that doesn’t feel quite right for him.”