"Ah, fire. So easy. So rewarding. So warm. Like a mother's hug. At least... how I'd imagine a mother's hug."
Played with most of the time, most notably for Family Thanksgiving, when Dan thought he was actually being accepted by Elise's family, only for it to be a elaborate ruse to gain his deviled egg recipe.
Dan being on the verge of tears upon learning that Hortense is getting married to Jeremiah Burger.
Everything Dan does in "The Family Thanksgiving". The way he seems completely unable to even comprehend the idea that people would enjoy his company, and almost seems to experience physical pain when he finds out he's just being used. He eventually passes out at having to choose between revenge and acceptance, and then is still rejected by the end of the episode. Doubly sad is the idea that Dan's issues go beyond simple anger issues, and that things like feeling wanted don't even compute for him.
There's also a blink-and-you-miss-it one one "Anger Management". Where Dan has to click a button when he sees an image that angers him. The image that sends him into a spiraling, button destroying rage? A flowery screen with the word "Mother" on it.
There's also the scene in Dan Vs. Dan, where the psychiatrist runs out of the room screaming after Dan tells him about his childhood. Although played entirely for laughs, if you consider what enraged him in "Anger Management" above, it's almost heartbreaking to imagine what kind of life the poor guy had growing up.
In "High School Reunion", he comments that his teen years were a time of constant ridicule, and that it only got worse at school.
The characters speeches in "The Family Cruise" when they're about to get sucked into a rip in space time in the sea. You really think they're gonna die there for a second.
The worst part has got to be that, just for a moment, Dan is accepted into the family. And thanks to the Reset Button, he will never remember it.
In "Summer Camp", young Dan is shoved out of the car at Camp Atrocious, and his bag thrown at his head before the car speeds off. From that, it might seem that, despite Dan's pleading, he may have been a miniature version of the violent, vengeful, vitriolic guy you've come to know the past three seasons. During the show, though, you learn that little Dan, while having a slight penchant for acting out, is not nearly the person he's become as an adult; in fact, he longs for affection and not only readily accepts Chris's friendship but attempts to play by camp rules even as the counselor continues to move the goalposts. Children want attention, and as child psychologists will tell you, if they don't get it for being good, they may attempt to get it by misbehaving, hence if Dan was misbehaving badly enough to aggravate his parents, it's very likely he just wanted them to pay attention to him. So, given Dan's desire for love and how he was kicked out of the car to start the show, the reasoning behind his entire personality — his childhood and mother issues, the reason why he felt abandoned by Elise's parents in "The Family Thanksgiving", his reaction to Hortense's marriage, even his possessiveness of Chris, becomes much clearer. Dan feels the world has abandoned and forsaken him, and, in many, many crucial cases... he's right.
Most of "The Neighbors" revolves around the fact that Dan is too paranoid to accept kindness. Dan is so traumatized from from the universe working against him, he refuses to believe anyone simply wishes him good fortune.
And right when the neighbors *do* start to like him, they go from laughing at his antics to absolute horror as his stories get more and more... um, *uncomfortable*. Dan either didn't know that normal people reacted like this, or thought it was OK because hey — someone actually liked him. He probably beat himself up about that one for a good long time.
Even though it's played for laughs, the ending to "Anger Management" stops being funny and just sad if you think about it for too long. At first, Dan is, as expected, not happy about going to anger management, especially when his instructor starts to take advantage of him. But when he starts to actually read and follow the steps in his workbook, Dan does a complete 180. Gone is the vengeful, hateful raging imp we have known throughout the show's run and instead we see a genuinely calmer, level-headed person. Sure, it freaked Chris out, but Dan seems to truly be much happier. When the instructor snaps near the end of the episode, Dan solves this problem though non-violent means, impressing the judge who put him into anger management to begin with, the only stipulation being he helps clean up the mess Chris made earlier, which Dan happily agrees to. Dan talks about how he's looking forward to his new life free from rage and muses about putting his energy towards more positive projects... only to get distracted by a squirrel and Status Quo Is God kicks in. Sure, we, the viewers, need Dan to be his angry self for the show to continue, but Dan loses a new lease on life. Honestly, having Dan conquer his anger issues once and for all would have made a nice series ending. Too bad the show got cancelled in the middle of season 3...
While the viewer gets some detail on Dan's Mother (and she probably being the reason he is the way he is today) very little is heard about his Father. It's revealed later this is because his parents are divorced, and as soon as they separated he never heard from his father again. The idea of talking to his Father again is so foreign to Dan that when Chris suggests to Dan to write his Father a letter, Dan thinks Chris is suggesting he mail the divorce attorney.
Dan having to give up Dennis at the end of "Parents." While adopting Dennis starts out as just a ploy to teach people he could be a good parent if he wanted to, Dan absolutely LOVES Dennis to the point of getting teary-eyed as soon as he meets him. Dan even refers to Dennis as his son for the rest of the episode, so when Dan's brushes with the law finally catch up to him, leading to Dennis being taken away, it's very heartbreaking to watch.
We're also able to see some reflections of how Dan was possibly treated as a child, reflected in his own parenting methods (until he meets Dennis, of course.) This includes making children do labor to earn respect, breaking their toys if they misbehave, and downright screaming at infants to shut up when they're crying. If this is the way Dan thinks parents should be, he had to learn it from somewhere.