It's the typical snowball effect, really. Every subscriber you gain could potentially tell their friends about you, and then they could tell their friends and it kind of just evolves from there - to the point where one day in the future, they'll all be like, "Dude, you watch _____ too? Awesome!"
It's definitely hard getting to your first 100, that's for sure. It takes so much work and even now, the majority of bigger YouTubers will tell you that it's such an uphill battle.
But it's doable! I'd say getting to the first 1,000 is the toughest of all, and after that (while it's obviously far from plain sailing) it's a little easier. But you do have to put the work in for it, regardless of where you are. It's easy to think you don't have to try as hard once you've reached your goal, but you've gotta keep giving your all and that'll keep the numbers rising.