Lucifer is an epic story with characters that have evolved from Season 1. While many fans love characters like Ella, Maze, and Lucifer, characters like Dan get less of the love. When the show started, Lucifer nicknamed Dan "Dan the Douche", and he seemed to fit the nickname. He wasn't showing up for Trixie (his daughter with Chloe), and he constantly doubted Chloe.
However, over time, Dan's character has grown, showing more nuances and interesting traits. There are things that fans love about Dan, and still, there are many reasons why he's not a fan favorite.
Love: His Protectiveness and Love for His Family
Initially, Dan didn't show up for likable and sweet Trixie as much as Chloe did. This changes, and he becomes a good father for Trixie. She is the key part of his family. Although he and Chloe are no longer together, he has retained a good relationship with her. He still sees her as family. Due to this, he is extremely protective of Chloe.
While he may act without knowing the full story, he often does so out of pure motivation to protect his family.
Hate: Instilled Doubt in Chloe
In the first season, Dan often second-guesses and doubts Chloe. In earlier cases, he encourages her to not make them complicated, to go for the easy answer. At this point, Chloe isn't as certain of her natural detective gift, and his opinion matters and causes her to doubt herself. Luckily, Chloe still follows her hunches despite Dan's doubt.
In time, Dan does come to trust Chloe. However, this is a trait that he has a hard time getting rid of completely. It results in a self-righteousness where Dan thinks he knows better for Chloe without asking for her point of view.
Love: Risked Vulnerability with Charlotte
When Mum (the Goddess, Lucifer's mother) brought Charlotte back to life, Charlotte was confused. Dan and Mum (housed in Charlotte's body) had a relationship. Dan, not knowing that the real Charlotte had been absent from her body, assumed that his relationship was the same. However, when Charlotte says she doesn't know who Dan is, nor seems to feel at ease with him, it hurts him. Later, after Charlotte and he begin to get to know each other again, he tells Charlotte, "I like you, Charlotte. So if you need time, I can wait. You're worth it."
Apparently, waiting isn't quite Dan's strong suit, so in an episode shortly after, he tells her that if she wants space, he'll respect that, but he needs to tell her where he stands. He decides to tell her what he wants and says, "I want you. All in. The whole deal." It's a vulnerable moment. He opens himself up to her, and it works. They become a great couple.
Hate: His Treatment of Ella
No doubt, fans may not have expected Dan and Ella to hook up. The fact that they hooked up was interesting, not bad. They provided solace and understanding to each other as well as the comfort that both needed at the time. What was bad happened afterward. Dan tried to distract Ella from finding out his dubious role on a recent hit on Lucifer. Then after that didn't work, he tried to cast doubt in Ella's theory that a cop had been involved.
Later, when she discovers the truth of his involvement and decides to help him by hiding his role, he gets mad at her. He wants to be punished. So, basically, Ella can't do anything quite right. Luckily, their interlude never resulted in a relationship.
Love: His Willingness to Try New Things
Dan comes across as pretty serious much of the time. This makes the moments when Dan is willing to try new things extra special. In Season 5, Dan tries to change, using self-help and improvement strategies. He borrowed a stack of self-help books from Linda. Also, he starts to wear an amethyst bracelet for chakra cleansing.
In previous seasons, Dan also does improv. He uses it to help him work through things. Dan also helps others use improv to heal and loosen up, such as when he brings Amenadiel to work through his issues. Unfortunately, each skit Amenadiel is in turns into one about his father. God, and how he may have to kill him.
Hate: Corrupt Cop Role
There are multiple times that Dan has been the corrupt cop, such as when he tried to kill Lucifer by arranging a hit on him. Also, he once teamed up with Maze, to work outside of the law and get forced confessions. Even in the beginning of the show, Dan was a corrupt cop.
Now, that Dan is becoming a more interesting and layered character, his corrupt cop role feels tired and flat. Hopefully, Dan will fight against his corrupt cop past and continue to grow into a more intriguing character.
Love: Sibling-Like Relationship with Lucifer
When these two get on better, they have great humorous and petty interactions like siblings sometimes do. Lucifer steals Dan's pudding, which is a big deal to Dan. The two both mock each other. In Season 3, the two watch over a teen, who is part of a murder investigation. At first, they think she's an innocent witness. They take her to an amusement park at her bequest. Both tease each other; Lucifer even makes shirts of sick-Dan on the roller coaster.
When their relationship is like this, teasing, it's something to love. Dan also gives Lucifer advice from time-to-time, proving that the two might be a bromance yet (that is, if Dan stops trying to kill or harm Lucifer.)
Hate: His Hatred and Distrust of Lucifer
At times, Dan sees all red with Lucifer. This leads him to think that he's doing a "good thing" by trying to get rid of Lucifer. When he thinks that Lucifer is human, he actually arranges a hit on Lucifer. When he finds out that Lucifer isn't human, he automatically believes that Lucifer is evil and must be stopped at any means possible.
Dan isn't always so good at listening to others during the best of times. But during the worst of times when he starts to hate and fear Lucifer, it brings out potentially disastrous conclusions--like his actions almost leading to Trixie's death.
Love: His Friendship with Amenadiel
This is a bromance fans can get behind. These two truly connect and respect each other. After all, Dan does bring Amenadiel to improv so that he can work through things. When Amenadiel becomes a father, it's Dan who gives him advice and helps him out. These two connect on many levels. They both are self-righteous, but they both are also extremely protective of their family.
The scenes with these two together truly shine and display the best parts of Dan. With Amenadiel, Dan doesn't come across as a "douche" but as a really good and wise friend.
Hate: How Easy He Is to Manipulate
While Dan has evolved throughout the series, he is still one of the easiest characters to manipulate. Michael proves this in Season 5, Part 1. In this season, Dan has finally seen Lucifer's true face, and he's terrified. He thinks that Lucifer is evil, and that he must save Chloe from Lucifer's influence. Michael tells Dan to shoot Lucifer, and Dan obeys easily. Normally, Lucifer would be dead because he was with Chloe and being with Chloe usually makes him vulnerable. However, things are changing, and luckily, Lucifer lives.
Still, after all of Dan's self-reflection and self-improvement in Season 5, this is a call back to the original Dan, the one who wasn't so popular with fans.