Years later, the things that caused you the most cognitive friction are the ones you're most likely to remember.
This reminded me of something Mark Penner and I once talked about several years back. I call it the Prodigy Point for lack of a better term. It's the point at which something you were originally "naturally good at" turns very, very hard. For instance, I've reached such a point in my drawing; up 'till this semester I'd tinkered happily along and learned how to turn out pretty decent stuff with no formal training. I was "naturally good" at art. I didn't need to think about it or work on it. It just happened to be that way.
Then this semester, BAM. Prof. Donis-Keller made me realize that what I'd really hit was a plateau of stagnation, and that to get any better I wouldn't be able to rely on my old "it just comes naturally!" habit any longer - for the first time, I'd have to struggle and study like everyone else - perhaps even harder than everyone else, since I wasn't used to struggling with drawing - and I was faced with the Prodigy Point (not that I was an art prodigy by any stretch of the imagination, but y'know). Do you keep zooming along on the plateau, or do you swallow your pride, set to work, and have a chance at becoming truly great at what you do, a greatness based on real skill and hard work, and not just talent and dumb luck?
I've hit that point with math, and have chosen to plateau on it for now (until winter break, when Rudin and I will meet again and I'll wrestle through Analysis). I've hit that point with music, and have been in a plateau for... I'm ashamed to say it, but almost 7 years. It's really cute when a toddler can pick out melodies on the keyboard by herself, but when the kid turns 20 and isn't much better, it's not cute any more. Man, it's hard to break out of that plateau - especially when your first "this is easy!" plateau might be at a way higher level than most people will ever reach through years of struggling. It's easy to rest on the laurels of arrogance. I definitely do it a lot.
You can hit the Prodigy Point with engineering, when you realize that messing with power tools in the school garage was awesome but you're going to have to hit the books and learn DiffEq to build something on the next level. You can hit it with programming, when you realize you've been a clever little hacker but can't wrap your mind around hardcore CS yet (learning Scheme was a major turning point for me in this). One of the things places like Olin and IMSA are good at is making a lot of folks - not all, but most - hit their Prodigy Points. Hard and repeatedly enough that you can't ignore their existence.
Do you love what you do because you honestly love it, enough to sweat and cry over it? Or do you love it because it's easy for you to be good at it and you enjoy coasting on the compliments? It's a turning point where something that was formerly easy becomes hard, and the way you deal with the Prodigy Point tells you a lot about who you are and what's really important to you.