Risk VS Reward
As an allegedly "normal" teenager, Dave clearly doesn't have a good skillset to become a superhero. He only trains for a few months at most, and doesn't have any particular talent for gymnastics or martial arts or anything especially athletic. He's just some guy. He took a HUGE risk going out "on patrol" and failed in a big way. In fact, "failed in a big way" is probably an understatement here.
But then, once he healed up a bit, he took the risk again. And this time, it paid off. He got accolades, he got status, he got the girl...
Now, granted, we're looking at a work of fiction here, but let's look at some reality for a moment.
I have a co-worker who does what he was trained for very well. I like to think I do what I was trained for pretty well too. But we were working on a project today, rushing against a deadline, trying to tag-team parts of the project to speed things along. At one point, he got to a portion that I had worked on originally, but I was busy with some other stuff so I asked him to just take that portion. He declined, citing an unfamiliarity with the work in question. Now, because we were working against a deadline and really needed to get it right the first time, that was a good call on his part.
But that's a not uncommon refrain from him. He regularly backs away from projects that he's not familiar with, even if he has plenty of time to sort through them and ask questions and figure things out. He's very risk-averse and prefers to stick to what he's already comfortable with.
By contrast, I try to lend as much help as I can even if I start going outside my comfort zone. I'm by no means a programmer, but I've dipped my toes in that pond so often that I've been mistaken for one on multiple occasions. And taking those risks of pushing myself beyond my known capacities have been extremely rewarding. Not always a monetary or physical reward, of course, but almost always an emotional or intellectual reward of some kind. From pushing myself to my limits, and then trying to extend a bit further.
Oh, there are times when I try pushing my limits and fall short. My current challenge is running a marathon, and I've had a few days of training where I could barely run half the distance I was supposed to. But that doesn't stop me. I get back out there and push myself again the next day. I did the same thing writing my book; some days were miserable failures and some days were more successful. And even though I haven't made a lot of money on my book, I still consider it a success because... well, I wrote a book. A lot of people die never having written the book they claim is inside them, but I did.
A lot of the comics I read growing up had a lesson along the lines of "the best things in life are the ones worth fighting for." While that was often used in the sense of winning something along the lofty ideals of freedom, honor and justice, I think it applies more practically to happiness, self-satisfaction and personal pride. Yes, you still have to fight for truth and liberty, but you have to fight for love and happiness more often and more readily.
Kick-Ass was fighting for justice while he was in costume, but Dave was fighting for a his sense of self-respect and self-worth all the time. And, ultimately, Dave took more satisfaction in the latter.