My take is that Dexter doesn't actually love blood but to say he hates it is also simplistic. He's obsessed with blood. It's the full cycle: obsession > anxiety > compulsion > temporary relief. Blood is the obsession. Killing is the compulsion. This is best illustrated with how he explains it to Deb in season 7:
"It starts with blood." "Blood?" "Yeah, the image of it. Trickling down the back of my eyelids. The trickle becomes a stream and then a flood. It fills me upā¦ all my empty spaces. But then the blood isn't red anymore." "What is it?" "It's black. And pressing. It feels like my head is gonna explode. The only way to relieve the pressure. Is to open the floodgates. And let it spill out."
He likes blood spatter analysis because its satisfying to catalogue blood. It gives him a sense of control. This is also why he likes killing. This need for control. Blood causes him immense anxiety. Killing relieves that anxiety.
Whats interesting is that he also uses killing to deal with anxiety that's caused by real tangible situations. In s7e1 Dexter is understandably anxious about Deb having caught him killing Travis Marshall. How does he deal with this anxiety? By killing Viktor Baskov. It's a maladaptive coping mechanism.
In the first episode of New Blood we see that while he's managed to resist giving into the compulsion the obsession and the anxiety are still very much there for Dexter. He tries to manage his anxiety through routine and when his routine is interrupted he looses control.
Blood is both the source of his anxiety and the thing that relieves that anxiety. It all comes back to his deep need to control it.
"Blood. Sometimes it sets my teeth on edge; other times it helps me control the chaos."
Also masking causes anxiety and Dexter feels like he can completely drop the mask when he kills. This reinforces the anxiety > killing > relief pattern.
