I won't lie. I really don't like Danny Pink.
This is something of a problem, since the latest episode, Dark Water, relied on the viewer having some affection for him. At least, we have to believe in his and Clara's true love forever.
But I really didn't believe in their love. I really didn't care when he suddenly, shockingly died of mundane causes. It all felt very "meh." And the reason, I think, is his relationship with Clara.
Although we know a few things about Danny, like his army background and the fact that he's not a PE teacher, he's defined almost entirely through his relationship with Clara, and his antagonistic relationship with the Doctor. And this means that all his most memorable moments and character traits are negative ones: being angry, being jealous, being controlling. Every single episode has shown their relationship as unhealthy and dishonest, making it hard to believe that Clara really does love him for all eternity, and would be willing to trap herself in a volcano to save his life.
He's been defined by Clara, but he not only doesn't have much development outside of her, he also acts as a leech on her character development. Their relationship undermines her at every turn, and any attempts to show her strengths are ruined the moment Danny appears on screen.
Take Flatline, for instance. In that episode, Clara was forced to take on the role of the Doctor, showed her talents for problem-solving and leadership, and learned a lot about what it means to be the Doctor along the way. But then the episode undermined all of its "Clara is a great, capable person" efforts with a few throwaway lines. The episode opened on Clara hurrying around the TARDIS, making sure she didn't leave so much as a toothbrush behind, because Danny would not approve. He was "fine" with her travelling with the Doctor, apparently, as long as she didn't leave a single scrap of evidence behind. Then later, we see her lying to Danny while in a life-threatening situation, because he actually hasn't given her permission to adventure with the Doctor, and she doesn't want him to know what's going on.
Apparently this is true love. Danny is jealous and possessive, Clara is lying to him, and she apparently needs his permission to time travel and have adventures. That's not romantic. That's creepy.
And if she's in a life-threatening situation when he calls, couldn't she just not pick up the phone?
In the Forest of the Night had similar problems. Danny was incredibly possessive, insisting that Clara "wasn't supposed" to be contacting the Doctor, and following her because he "decided it was best not to leave her alone... with him...."
All of their relationship drama is built around the Doctor. Initially, it's about her struggles to hide her adventures from him. And when he does find out, his reaction isn't "cool, time travel! Adventure!" but "you shouldn't be doing this." And so she lies to him. She continues to hide her adventures, because he'll be angry, he'll be possessive, he'll be jealous.
And this causes majors problems for the latest plotline. Because we have to believe that Clara will sacrifice everything for Danny. That she'd be willing to die if she can't be with him, that she'd willingly destroy the Doctor for him, that he is it for her. We have to believe this, even though the only thing we know about their relationship is that he's kind of controlling. We have to feel it, despite the fact that all I feel is that he should go away.
And it adds a disturbing tone to everything that happens. At the beginning, it seems that Clara is planning to finally tell Danny everything, and has post-it notes set up to help her. But before she tells him about her adventures, she makes sure to insist that she will never love anyone again, ever, even if they break up. Because she's basically betrayed him by having an amazing life, right? The problem is definitely not with him.
An epic romance story for the ages, am I right?
If we wanted to give the writers a lot of credit, we could say that they were attempting to flip genre fiction stereotypes, where the male character has adventures and the female love interest is defined entirely by her relationship to him, and, perhaps, her disapproval of his constant disappearances. It could be analysed that way. But I think that's giving the writers far too much credit. Danny is mostly defined by Clara because the show failed to dig deeper and give him a compelling personality in his own right, and he's possessive because that's a pretty default trope to fall back on for some drama.
Basically, Doctor Who needs to get some female writers. And some better male writers. And just think more about characterization in general. Because although it's been a good season, with a lot to commend it, it's unfortunate when a significant character dies, and the response is simply "meh."