I have a question :) In my writing courses back in college everyone told me that my characters needed reasons to be as weird as I wrote them rather than "just because" or "that's how they are" and "there are real people that are like this" which are my rea

I’m going to answer this question in two parts:

1.) Nobody’s personality develops in a vacuum. While many of our character traits are inherent to us (introversion vs. extroversion, anxious vs. laidback), our more extreme/complex character traits are the product of numerous psychological, social, and environmental factors. 

When developing characters, you do have to take that into account. “Weird” characters typically A.) Don’t understand societal norms (Which is why you get so many “weird” characters who arguably fall in the Autistic spectrum - not saying I condone this, btw, just saying that this is something that happens), or B.) Have actively chosen to not give a fuck about societal norms. (Luna Lovegood would be a good example of the former; Willy Wonka (movie version) would be a good example of the latter.) 

It’s very clear that Luna Lovegood doesn’t think the way that most people think. She has a unique perspective on the world around her, and this is obvious in everything she does. This unique perspective is likely the product of her upbringing - her dad is just as “unique” in his vision as she is, and her mother died as a result of magical experimentation. (Which Luna witnessed - let that sink in.) 

Willy Wonka is an odd person, but by the end of the film you see that his oddity is his way of rejecting society. He hates people. He distrusts them to the point that he designed a strange and cruel experiment to test/torture children to determine their worthiness to inherit his chocolate factory. It may seem like reading into things, but it does help make sense of his character and makes him a bit more than just some goofball on a sugar high. 

The thing is, in real life you don’t see the reasons people act strange, but that doesn’t mean there isn’t  a reason. It just means that you aren’t privy to that information. Which brings me to part 2 of this answer:

2.) Truth is stranger than fiction. This term gets thrown around a lot, but what it really boils down to is this - In real life, random things just happen, randomly. People get in their car to drive home and get killed in random accidents. People do weird things on a whim, just for fun. 

But life is life, and stories are stories. And when you’re telling a story, everything needs to work together in some form or fashion to complete the narrative. There has to be a point (unless you’re writing very specific forms of nihilstic, existentialist, postmodern, etc. literary fiction, but I digress.) 

So, getting back to what you said - maybe your character dresses like the devil in July. But why make it random when you can make it something more?

“What are you wearing?” said Anastasia. “Are you nuts?”

“You’re wearing a costume. Why can’t I wear one too?” said Katie as she adjusted her horns. 

“This is my uniform. For my job. I’m not traipsing around like Lucifer on acid.”

“It’s still a costume. You’re pretending to be something you’re not. And if the world is telling me I have to wear a costume to get by, at least I’m going to choose the costume I wear. Now, if you’ll excuse me I have mischief to spread and innocents to corrupt.”