Through much of his life, Baker Mayfield’s potential on the football field has been doubted. The 6-foot-1, 220-pound quarterback was a walk-on with two different football programs before ultimately starting at Oklahoma and winning the Heisman Trophy this season.

In an in-depth thank you essay published in The Players’ Tribune on Thursday – the third day of the NFL combine – Mayfield details his journey from a “stout” 5-foot-2, 140-pound freshman in high school to a projected top-10 NFL Draft pick.

Mayfield explains – and as we’ve seen in previous mock drafts – he is often undervalued because of his size, or lack thereof. And that’s been a major contributor to his sometimes controversial demeanor and actions.

He wrote, via The Players’ Tribune:

I’ll get asked a lot about my approach to the game and my behavior on the field. Why did I make a big display after Kansas refused to shake my hand after the coin toss or when I heard Baylor’s players talking trash before we played against them?

Basically, Why the attitude, Baker?

The truth is, I’m always going to remember what it was like being that kid who was too small to even be given a second look. I’m going to remember what it feels like to be doubted and how amazing it feels to overcome that doubt. In order to be my best, I need to play with an edge. I’ve always been energetic, and what I’ve found is that by showing my emotions, my teammates bring out their emotions as well, which can take everyone’s performance to another level.

Sometimes my passion has been mistaken for immaturity — and there definitely were some moments when I was out of line that I ultimately apologized for. But these past four years, I was living out my lifelong dream, and I’m proud that I did it on my terms.

Mayfield also explained how he would sneak onto the field at night for some extra reps with imaginary teammates and how former Sooners head coach Bob Stoops’ retirement affected him.

He also thanked his teammates, Stoops, current coach Lincoln Riley and Oklahoma fans, who have “a special place in my heart.”

Read the full essay at The Players’ Tribune.