When Michael Myers murdered his sister Judith on Halloween night, he was dressed up in a clown costume. You might wonder if that means anything. It does, because that makes him a Monster Clown, and clowns have had a long history of existing and behaving outside the rules. That's right, Michael is essentially The Joker of slashers, engaging in random killings and behaviour from childhood!
Plus, pyschopaths and sociopaths also have a history of behaving outside the rules, and Michael is definitely one or the other.
From part 2: several times throughout the movie, Michael's eyes are clearly visible in close up through his mask. At the film's climax, Laurie shoots Michael's eyes out.
While people usually mock the sequels (especially 4 & 5) for not having good masks for Michael, it actually makes a lot of sense in-universe. After the 78 murders, the original mask was likely put out of production, given the murders committed while wearing it. So it makes perfect sense that there wouldnt be a proper mask anymore!
The reason why Michael always wears a white mask is because he feels that it's his true face (think Rorschach from Watchmen). Underneath his mask, Michael is shown to be normal-looking, however, he's a textbook sociopath with no real emotions, and wears a mask with a blank, expressionless appearance. He also wears the mask to convey a feeling of power. With the mask on, he feels unstoppable, however when he loses it, he feels weak; this can be seen when he's unmasked after killing Judith and during his fight with Laurie. He looks shocked and he stops what he's doing. This theory also explains Michael's obsession with Halloween. Remember that Michael has been locked up since he was six, so theoretically, he may have the mind of a six year old. His delusions of reality has him still thinking that monsters are real, and when he dresses up as one, he becomes one for real, as Halloween is the supposed time where ghosts, ghouls, goblins, and shapes roam the night. This is also why he scrawled "Samhain" in blood in the second film; he probably learned about the history of Halloween in school and became obsessed with it, as it gave him the idea that it was an outlet to indulgence in his sociopathic tendencies by killing someone and getting away with it; it didn't work out, since he got put in an asylum in the end.
When Michael escapes from Smith's Grove in the first film, he murders a tow truck driver in order to steal the man's coveralls. He is then shown using coveralls as his regular outfit throughout the franchise. The reasons why Michael keeps using coveralls is because since there are repair shops, gas stations, and truck stops here and there, giving him several convinent locations for him to steal a pair of coveralls whenever he needs a new pair; coveralls are usually unisex and made so one size fits all, allowing a comfortable fit regardless of who Michael mugs; lastly, the coveralls provide camouflage for Michael. When he's roaming around Haddonfield, no one pays close enough attention to him since he looks like a blue-collar worker out on a break and despite being masked, Michael's mask looks so much like a real face it looks normal from a distance. The dark coloration of the coveralls also help Michael blend in the environment during nighttime.
After watching the latest film, Halloween (2018), another theory emerges; Michael wears the same mask and coveralls because he wants to relive the feeling from his original killing spree.
When Lynda is killed in the first film, her final thoughts must be that Bob, her boyfriend, is the one attacking her. She never saw Michael Myers, and just assumed that he was Bob in a sheet ghost costume playing a trick on her. She then turned her back to make a call, and Michael murdered her with the phone cord before she saw him. I suppose it doesn't make a tremendous difference either way, but still, the poor girl thought it was her boyfriend strangling her to death.
When Michael escapes in the first film, he lets out a bunch of patients who acted like harmless, drugged-out zombies. But Michael can't be the only murdering psychopath in the place, right?.
In the 5th movie, we see Jamie getting rocks thrown through her window, because of what she did, and because shes Michael's niece. Sucks for her, but think for a second. What did the people do to Michael's parents after he murdered Judith, and Laurie after the 78 murders? Laurie might have lost some of the heat given she didnt know and killed Michael, but just imagine what Mr. and Mrs. Myers might have gone through.
In the 1978 film, when Laurie tells Tommy and Lindsey to go to the MacKenzies' house and have them call the police, why didn't she just go with them? What was keeping her there, anyway?
She's exhausted, injured, and thinks that Michael's dead and no longer poses a threat to her. She probably figured that the best thing to do was stay put and rest until the police and ambulance got there.
How does Michael know how to drive a car in the first film, given he's been incarcerated in a mental institution for 15 years, since he was six?
This was actually brought up in the first film itself, with Loomis making a guess that "someone around here must have given him lessons." But, that proposes a bigger question: why would a member of the staff give Michael driving lessons, even more so, how would they be able to without even Dr. Loomis knowing about it?
I think Loomis's lessons line was more sarcastic than anything. Michael likely just paid very close attention to the driver's actions when he was in a car.
Another possibility: he watched his parents drive when he was little. Parents do drive kids around, and he may have figure out what does what at an early age just by watching them.