How can you tell Dave is motivated by other people’s opinions of him? Give a reasonable explanation in YOUR own words. Please and thank you.

STORY: HEARTBEAT, Short Story by David Yoo

For the rest of fall, I did countless of push-ups and curled free weights until I couldn't bend my arms. I got ridiculously strong and defined, but I wasn't gaining weight. I wanted to be thicker. I didn't care about getting stronger if nobody could tell. I did research, and started lifting heavier weights at lower reps...

Dave says, “I didn’t care about getting stronger if nobody could tell.”

What does this sentence prove/show?