Midoriya Izuku wasn't born with an amazingly powerful or incredibly useful Quirk.
He wasn't Quirkless or anything, he was born with a Quirk. It's just..., it wasn't one most people could see being in the hero scene.
Bakugo, his best friend before they got their Quirks, laughed at Izuku for his Quirk. "I knew you're Quirk couldn't beat mine but it's just so bad!" It didn't help that his peers, especially the girls tried to avoid him.
You see, Izuku's Quirk is called Girl Control. (Izuku tried desperately to call it something more heroic sounding but he really couldn't) Girl Control allows Izuku to sway people opposite his gender on varying levels, depending on how they view him. For example, a girl who doesn't know Izuku wouldn't be swayed in any way; a girl who thinks Izuku is a pretty cool person is able to be affected in their thought process but otherwise, they can still think independently while girls who hold Izuku in high regard can be either strongly swayed or taken over control.
In fact, the day Izuku first activated his Quirk was the day his mom bought him a limited-edition All Might poster after he accidentally broke several plates. Once Inko was freed when Izuku finally fell asleep from being allowed to stay up late, she made sure to cut his All Might watching time for a whole week.
As the years went on, the process stayed relatively the same; Bakugo looked down on him for being born with such a weird Quirk and the girls fled from his very presence. After all, who'd want to befriend someone who could easily take advantage of you for just that? Early in his life, Izuku never really cared too much, still being caught up in the cootie conspiracy and avoided girls anyways but it was in his first year of middle school that made him realize how much he hated his Quirk. It was a new year, a new school with new students! And Izuku started going through puberty.
Izuku's puberty wasn't actually that bad; small amount of pimples that left pretty quickly and his voice changing too fast for anyone to make fun of voice cracks. No, the physical part of puberty wasn't bad at all. The hormonal part...
On Izuku's first day of middle school, a girl new to the city approached him with an offer for friendship. This was the first time a girl his age socialized with him, coupled with his growing interest in girls as a result of puberty and her cute yet sexy looks, this girl was his first crush. But it all came crashing down when a clique of girls from his elementary school butted in.
"Ew, get away from him!" "He'll force you to kiss him!" "He's a pervert, a freak!" "His Quirk can control you if get too close, it's a trap!"
The second his crush heard that, she looked between the girls and Izuku before saying "Oh! Sorry. I-I just, I thought you looked nice to hang out with but-" and she shook her head and left him, a shattered heart left in her wake.
After that, the clique laughed at him for feeling like he got what he deserved and went to console his latest "victim" which led to Izuku's very first crush becoming another tormenter. To top it all off, Bakugo came to mess with him again later that day.
"Deku, that Quirk of yours isn't made for a hero. Don't you get it? It's a villain Quirk! And you should remember what heroes do to villains, right nerd?"
The first day of middle school was then officially dubbed "The Day Izuku's Heart Broke" as for several weeks since that day, Izuku never responded in school which resulted in low marks. That was also the day Izuku swore to never pursue love again since the second he even dipped his feet into that pool, he was torn to shreds mercilessly by the sharks known as females.
Izuku still felt that there was a chance for him. That, through all odds, everyone would look at him, hear his name and know that a hero was there to help. However, no one else believed it; believed him when he said he'd be a hero. Izuku had nobody to believe in him, not even his own mother.
Sure, Inko dismissed his Quirk playfully when he used it on her and knew that her son was too precious to be villainous in any manner. She never believed that Izuku deserved the treatment he received from his peers and tried many a time to stop it. He never deserved the discrimination; he was a legitimately good person! However, in Inko's eyes, Izuku should never become a pro hero.
Don't get her wrong, her heart swelled whenever Izuku stepped in to help her however he could, just like a hero. She just couldn't bear the possibility of Izuku standing proudly in front of a group he rescued, only for him to be scorned. Not only that, his Quirk's nature basically renders him as Quirkless in combat. After all, it's not like there's an abundance of female villains, much less ones who'd be looking up to him.
And so Izuku stands, looking at the path of a hero from the sidelines while many who were walking down it spit upon him. What could Izuku possibly do to make himself a hero with such a Quirk?